The ovarian cycles of 48 culled dairy cows were monitored by assaying plasma progesterone concentrations. Twenty-four cows received a subcutaneous implant of 300 mg trenbolone acetate (Finaplix; Hoechst) at the beginning of the study. Of the implanted cows, two were pregnant, six continued to cycle although their peak progesterone concentrations were significantly lower than in control cows (5.65 +/- 0.71 compared with 8.19 +/- 0.47 ng/ml; P less than 0.01). Prolonged periods (13 to 92 days) of low progesterone concentrations (less than 1 ng/ml) occurred in 12 of the implanted cows. In six of seven cows in which normal cycles had not resumed by the time of slaughter two to five ovarian follicles of diameter greater than or equal to 10 mm were found post mortem. Persistent luteal function (greater than 35 days) occurred in the absence of pregnancy or gross uterine pathology in five of the implanted cows, two of which had recovered spontaneously by the time they were slaughtered. There was no difference in plasma luteinising hormone concentrations between the implanted and control cows. It is concluded that trenbolone acetate affected the ovarian cycle of the cows in several ways through changes other than the modification of tonic luteinising hormone secretion.