Since stress has been shown to influence prolactin release in sheep, the secretion of this hormone during the oestrous cycle of the ewe was re-evaluated. At intervals of two hours, for a total period of 40 hours, blood samples were obtained from oestrous, inter-oestrous and anoestrous ewes, and from wethers. An acute release of prolactin was elicited in all the ewes, irrespective of their reproductive state. Prolactin levels in excess of 1000 ng/cm3 were at times measured in the cycling and anoestrous ewes. Relatively smaller quantities of prolactin (25-400 ng/cm3) were secreted in the wethers. A typical pre-ovulatory luteinizing hormone (LH) peak was measured shortly after the onset of heat in the oestrous ewes. The results are discussed in relation to the reputed existence of an oestrous-related prolactin surge in ewes.
