Administration of ovine antiserum containing antibodies against synthetic LH-RH to spayed female ferrets caused an immediate fall in plasma LH concentration and by 2 h after injection circulating levels had declined by 70--80%. A further decline of 50% was observed on Day 2, but by Day 4 a return to normal levels had begun and control values were regained on Day 6. In contrast, the fall in plasma FSH concentration was delayed, with a significant reduction being first observed on Day 2 and minimal values occurring on Day 4. Control values were regained by Day 7. The results accord with the concept that a single hypothalamic releasing factor controls the tonic secretion of both pituitary hormones. However, the slow change in FSH secretion implies that the mechanism for FSH release may be more autonomous than that for LH.