Diurnal profiles of the content of immunoreactive somatostatin (IRS) in the male and female rat pineal, Harderian gland and retina have been studied. Supplementary experiments have been performed to elucidate a possible effect of infradian cycles, namely estrous and seasonal cycles, and continuous dark on IRS concentration. Results demonstrate that the IRS content in the rat pineal gland, Harderian gland and retina is submitted to diurnal variations, but not under all studied conditions. A sexual dimorphism exists between male and female animals: male rats showed higher IRS content in pineal (103.8 +/- 4.7 vs 32.3 +/- 1.8 pg IRS/gland), but lower in retina (1,362.1 +/- 82.7 vs 2,176 +/- 102.2 +/- pg IRS/mg of protein) and Harderian gland (10.3 +/- 0.8 vs 30.6 +/- 3.5 pg IRS/mg of protein). Additionally, seasonal differences appeared: in male and female animals pineal IRS content was lower in spring than in November. This decrease also appeared in female retina IRS concentration. Estrous cycle did not seem to change IRS content in the three studied tissues. Finally, pineal IRS rhythm persisted after continuous dark for a week. These results demonstrate, in the rat, sexual differences in the IRS content of the various tissues studied and suggest a physiological role for somatostatin, possibly related to seasonal adaptation.