The changes in the nucleus preopticus (NPO) and nucleus lateralis tuberis (NLT) of the catfish. Clarias batrachus, were studied after electrical stimulation of the optic nerve. The aldehyde fuchsia (AF)-positive neurosecretory material from the entire NPO was depleted after a 10-min stimulation as revealed from the bulk-stained preparations and tissue sections. However, a 3-min stimulation was sufficient to cause significant hypertrophic changes in the neuronal nuclei (p<0.005 in the ventral neurons; p < 0.01 in the dorsal neurons). In order to evoke significant stimulatory changes in the cell nuclei of the NLT, a longer period of stimulation than that for the NPO was essential. Similarly, stimulations of different durations were required to induce significant hypertrophic changes in the pars anterior and pars posterior of the NLT. Based on these observations, the possible existence of an afferent pathway from the optic nerve to the hypothalamic neurosecretory nuclei is suggested.