It has been hypothesized that nitric oxide (NO) may act as a 'retrograde messenger' in the CNS, mediating intersynaptic communication in the context of neural plasticity during memory formation. To test this hypothesis the effects of the competitive NO synthase inhibitor N-nitro-L-arginine (NARG) on memory retention has been studied in a one-trial passive avoidance task in the day-old chick. Intracerebral injections before training into the intermediate medial hyperstriatum ventrale (IMHV), an area that is of crucial importance in learning in the chick, produced amnesia in this task when tested at various time points from 30 min to 24 h after training. Time- and dose dependencies of NARG when injected i.p. or i.c. were evaluated. Injection into one IMHV alone (left or right) proved to be sufficient to produce amnesia. Diffusion of NARG into the untreated hemisphere was ruled out by injecting it with L-arginine, which competes with NARG and prevents inhibition of NO synthase. Additional tests showed that the amnestic effect is not due to state-dependent learning, nor to interference of the drug with general motor ability or motivation.