Ovulatory activity in Black-Belly ewes maintained under two different photoperiodic conditions. Two groups of Black-Belly ewes were maintained in light-proof rooms and subjected to either simulated temperate photoperiodic conditions (TE solid line closed circles, 12 ewes, 8 to 16 hours of light per day from winter to summer solstice) or simulated tropical photoperiodic conditions (TR dashed line open squares, 12 ewes, 11 to 13 hours of light per day from winter to summer solstice) for 25 months. Ovulatory activity was assessed by once-weekly determinations of plasma progesterone by radioimmunoassay. Results are expressed as the percentage of ewes experiencing at least one ovulation per month. Significant differences between groups are indicated.