diff --git a/bin/cuirass.in b/bin/cuirass.in
index b41a713..7df5ddb 100644
--- a/bin/cuirass.in
+++ b/bin/cuirass.in
@@ -7,6 +7,7 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
 !#
 ;;;; cuirass -- continuous integration tool
 ;;; Copyright © 2016 Mathieu Lirzin <mthl@gnu.org>
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;;
 ;;; This file is part of Cuirass.
 ;;;
@@ -34,6 +35,7 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
 
       --one-shot            Evaluate and build jobs only once
       --cache-directory=DIR Use DIR for storing repository data
+  -L  --load-path=DIR       Prepend DIR to Guix package module search path.
   -S  --specifications=SPECFILE
                             Add specifications from SPECFILE to database.
   -D  --database=DB         Use DB to store build results.
@@ -48,6 +50,7 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
 (define %options
   '((one-shot                         (value #f))
     (cache-directory                  (value #t))
+    (load-path      (single-char #\L) (value #t))
     (specifications (single-char #\S) (value #t))
     (database       (single-char #\D) (value #t))
     (port           (single-char #\p) (value #t))
@@ -68,6 +71,8 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
          (%package-database (option-ref opts 'database (%package-database)))
          (%package-cachedir
           (option-ref opts 'cache-directory (%package-cachedir)))
+         (%guix-package-path
+          (option-ref opts 'load-path (%guix-package-path)))
          (%use-substitutes? (option-ref opts 'use-substitutes #f)))
       (cond
        ((option-ref opts 'help #f)
diff --git a/bin/evaluate.in b/bin/evaluate.in
index c8a45ce..0508783 100644
--- a/bin/evaluate.in
+++ b/bin/evaluate.in
@@ -8,6 +8,7 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
 ;;;; evaluate -- convert a specification to a job list
 ;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016 Mathieu Lirzin <mthl@gnu.org>
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;;
 ;;; This file is part of Cuirass.
 ;;;
@@ -32,7 +33,7 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
 
 (define* (main #:optional (args (command-line)))
   (match args
-    ((command load-path cachedir specstr database)
+    ((command load-path guix-package-path cachedir specstr database)
      ;; Load FILE, a Scheme file that defines Hydra jobs.
      (let ((%user-module (make-fresh-user-module))
            (spec         (with-input-from-string specstr read))
@@ -58,6 +59,8 @@ exec ${GUILE:-@GUILE@} --no-auto-compile -e main -s "$0" "$@"
                  (exit 1)))
          (parameterize ((%package-database database)
                         (%use-substitutes? (assoc-ref spec #:use-substitutes?)))
+           (unless (string-null? guix-package-path)
+             (set-guix-package-path! guix-package-path))
            ;; Call the entry point of FILE and print the resulting job sexp.
            (let* ((proc-name (assq-ref spec #:proc))
                   (proc    (module-ref %user-module proc-name))
diff --git a/src/cuirass/base.scm b/src/cuirass/base.scm
index 8a14ddc..2fde19b 100644
--- a/src/cuirass/base.scm
+++ b/src/cuirass/base.scm
@@ -1,6 +1,7 @@
 ;;; base.scm -- Cuirass base module
 ;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2016 Mathieu Lirzin <mthl@gnu.org>
+;;; Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;;
 ;;; This file is part of Cuirass.
 ;;;
@@ -20,6 +21,7 @@
 (define-module (cuirass base)
   #:use-module (cuirass database)
   #:use-module (cuirass utils)
+  #:use-module (gnu packages)
   #:use-module (guix derivations)
   #:use-module (guix store)
   #:use-module (ice-9 format)
@@ -33,7 +35,9 @@
             evaluate
             build-packages
             process-specs
+            set-guix-package-path!
             ;; Parameters.
+            %guix-package-path
             %package-cachedir
             %use-substitutes?))
 
@@ -114,6 +118,7 @@ if required."
                            (string-append (%package-cachedir) "/"
                                           (assq-ref spec #:name) "/"
                                           (assq-ref spec #:load-path))
+                           (%guix-package-path)
                            (%package-cachedir)
                            (object->string spec)
                            (%package-database)))
@@ -174,3 +179,20 @@ if required."
       (db-add-stamp db spec commit)))
 
   (for-each process jobspecs))
+
+
+;;;
+;;; Guix package path.
+;;;
+
+(define %guix-package-path
+  ;; Extension of package modules search path.
+  (make-parameter ""))
+
+(define (set-guix-package-path! path)
+  "Use PATH to find custom packages not defined in (gnu packages ...)
+namespace or not already present in current Guile load paths."
+  (%package-module-path (cons path (%package-module-path)))
+  (%patch-path (cons path (%patch-path)))
+  (set! %load-path (cons path %load-path))
+  (set! %load-compiled-path (cons path %load-compiled-path)))