High densities of atriopeptin-immunoreactive fibers and of highly specific and selective atriopeptin receptor sites are present in brain areas involved in animal behavior. The possible influence of these peptides on behavior was thus investigated in adult rats. The intracerebroventricular injection of atriopeptin II modified male sexual behavior (reduction in mount latency) at the dose of 5 micrograms/animal; lower and higher doses were ineffective. Open-field behavior was also modified by i.c.v. atriopeptin II at the doses of 5 and 10 micrograms/rat, which induced an increase in the number of external and internal crossings and of external rearings. Finally, in fasted rats, atriopeptin II, at the dose of 10 micrograms/rat, significantly increased the amount of food intake 30 and 60 min after injection. These findings indicate that atriopeptins may modify different animal behaviors.