The effects of alpha-MSH on ovarian progesterone (P) and estradiol (E2) release were investigated. To this purpose, different doses of alpha-MSH were administered to prepubertal ovaries incubated under the following conditions: ten 5 min-pulses of perfusion medium (PM) plus LH at 160 ng/ml each one followed by a 25-min period of PM. The medium of the treated group contained alpha-MSH (0.1, 1, 5, or 10 microg/ml, in saline solution) and the control group saline solution. PM was assayed for P and E2 release. The area under the curve, pulse amplitude and width, and basal hormonal release were calculated for each hormone profile. With respect to P release, an increase in the areas under the curve could be observed in ovaries treated with 1, 5 and 10 microg alpha-MSH/ml. This increment generally coincided with an increase in P pulse amplitude and/or basal P release. The P pulse width was not modified. No significant variations were observed in the parameters under analysis with regard to E2 release. It is concluded that alpha-MSH administered to prepubertal ovaries could exert a physiological role through stimulation of P release.