The exposure of Aplysia brasiliana to dilute seawater (90 and 80%) caused an increase of the relative weight, which returned to the original values after a few hours. Both osmotic and chloride concentrations of the hemolymph decreased on exposure to 80 and 90% dilute seawater, and after 3-h exposure there were no differences between the hemolymph and external media osmotic and chloride concentrations. In contrast to the clear regulatory capabilities reported for A. californica, A. brasiliana cannot maintain the osmolality of its body fluid in dilute media. In A. californica, osphradial receptors and neuron R15 are apparently involved in this regulatory mechanism. Perfusion of osphradium of A. brasiliana with dilute seawater (95-80%) did not affect electrical activity of the bursting neuron R15; perfusion with 70 and 60% seawater caused a transient increase in the duration of the quiescent period. In contrast to the model established for A. californica, in A. brasiliana no relationship was found between exposure of the osphradium to dilute media and electrical activity in neuron R15, which is in accordance with the lack of an osmoregulatory mechanism in this species. Such differences may reflect inherent differences in salinity tolerance between the two species.