Effects of progesterone treatment on basal and LH-RH-induced plasma LH concentrations in anoestrous and ovariectomized sheep

Anoestrous and ovariectomized sheep were treated with progesterone-containing implants or left as controls. Plasma LH concentrations were decreased by progesterone treatment (P less than 0.025) and increased by ovariectomy (P less than 0.001). When the ewes were treated with 0.25 micrograms LH-RH, the LH response was reduced (P less than 0.025) in the progesterone-treated anoestrous ewes but not in the progesterone-treated ovariectomized sheep. These data demonstrate that low concentrations of progesterone, acting together with another ovarian hormone, can markedly reduce the pituitary response to a small dose of LH-RH.