The intermediate and medial part of the hyperstriatum ventrale (IMHV) is a memory system in the chick forebrain in which certain learning-related changes occur after imprinting. We have enquired whether NADPH-diaphorase, a presumed marker for nitric oxide synthase, is present in sections through the IMHV of chicks, either trained with an imprinting stimulus or dark-reared. Very few NADPH-d-positive cells were found in the IMHV (0.37 +/- 0.07 S.E.M. cells per 0.3 mm x 0.9 mm sampling frame), in contrast to the palaeostriatum augmentatum (PA) (19.47 +/- 0.77). Some stained cells in the PA were closely associated with blood vessels. The results do not support the hypothesis that learning-related changes in the IMHV depend on nitric oxide acting as a retrograde neuronal messenger.