Individual 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 ewes 1-12 at the bottom of the figure, 8 to 16 hours of light per day from winter to summer solstice) or simulated tropical photoperiodic conditions (TR ewes 13-24 upper part of the figure, 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. One line represents one ewe. A single dot represents one progesterone measurement above 0.75 ng/ml of plasma and is considered as indicative of the presence of at least one corpus luteum. All ewes were present until the end of the experiment.