Jugular blood samples were obtained from 8 mares at 5- and/or 20-min intervals for 2 to 5 days during various phases of the oestrous cycle for plasma LH determination. An episodic release pattern was observed in 1 of 3 mares sampled during the ovulatory period. One mare had one secretory burst and the other mare had several periods of fluctuating plasma LH concentration. During dioestrus, episodic secretions were observed in 2 mares sampled 11 to 13 days before and, in 1 mare, 9 days after ovulation. During the 2 to 5-day period before ovulation, episodic secretion was not observed (3 mares) but plasma LH concentrations fluctuated as much as 6 ng/ml during a period of 3--4 h. Daily plasma samples were obtained form 10 mares (1--8 oestrous cycles/mare) during which 22 single, 18 double and 2 luteal-phase ovulations occurred. Dioestrous ovulations were accompanied by small increases in plasma LH (1--4 ng/ml), but many similar increases in LH were not accompanied by ovulation. No significant differences in secretory patterns were observed between single and multiple ovulations. In one mare, 4 ovulations occurred in the presence of a prolonged luteal phase; 3 were accompanied by increasing LH concentrations and the other occurred when LH was at a low concentration.