The mollusc, Lymnaea stagnalis, has been used as a model to study the mechanisms of nitric oxide (NO)-dependent processes in the CNS. Putative NO-containing neurones in Lymnaea are localized in the buccal ganglia, predominantly in areas where sensory neurones known to regulate feeding are found. The NO-generating substance, S-nitrosocysteine (S-NC, 5 x 10(-5)-10(-3 M) activates feeding movements of the buccal mass and modulates the activity of buccal motoneurones. An inhibitor of NO synthase, NG-methyl-L-arginine (10(-4) M) decreases the frequency of background buccal movements and has opposite effects to S-NC on the buccal motoneurones. We suggest that NO is a messenger in the CNS of Lymnaea and may be involved in coordination of feeding motor patterns.