Measurements were made of the plasma concentrations of testosterone, progesterone and oestradiol-17β during the oestrous cycle of the ewe, and of the plasma concentrations of testosterone and progesterone during the oestrous cycle of the heifer. The maximum concentration of progesterone in the heifer was nearly twice that in the ewe. The maximum concentration of testosterone was similar in both species and occurred at the onset of luteolysis; in the ewe the peak concentration of 19·5 pg/ml plasma was significantly greater (P < 0·05) than the concentrations of testosterone measured during the remainder of the oestrous cycle.