A radioimmunoassay for ovine LH is described in which the plasma values are lower than those reported in most other radioimmunoassay studies. Simultaneous measurements of estrogen and LH in the ovarian venous effluent of sheep were performed by radioimmunoassay. Levels of both hormones were elevated during the follicular phase. The first rise in estrogen levels was detected 1 day before the LH rise. The measurement of LH in the peripheral and ovarian venous blood collected from the same sheep at the same time (± 5 min) indicated no significant lowering of LH in the ovarian venous effluent when compared to jugular levels.