Eight mares were observed clinically for 14 months to detect oestrus and ovarian activity, and assays of urinary oestrogens and plasma progesterone were performed. Cyclical ovarian activity occurred during 60% of the experimental period. Persistent luteal activity was found over 20% of the period and for the rest of the time activity was absent. Cyclical activity was characterized by the alternation of periods of low and high progesterone levels corresponding to the follicular and luteal phases of the ovarian cycle. During the follicular phase, urinary oestrogen levels increased to a maximum of 463 +/- 296 ng/ml 2 days before ovulation. The duration of the interval from complete luteolysis to ovulation (6-3 +/- 2-7 days) was correlated with oestrone peak values (r = 0-44) and with oestrone levels at the beginning of the low progesterone phase (r = -0-36). High oestrogen levels were recorded during persistent luteal activity which occurred more frequently in summer. Ovarian inactivity was more frequent in winter and was characterized by very low progesterone and low oestrogen (less than 50 ng/ml urine) levels. Some individual and seasonal effects were studied.