Mean plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels and their pulse frequencies and the size of the largest ovarian follicle increased during spring in 12 mares studied twice monthly from deep anoestrus to the occurrence of the first ovulation of the breeding season. Mean FSH levels were reduced significantly in deep anoestrus and when a pre-ovulatory follicle was present, whereas mean LH levels were highest close to ovulation. Five of these 12 research mares and 20 of 40 maiden or barren Standardbred mares at a commercial studfarm were given daily intramuscular (i.m.) injections of 150 mg progesterone in oil for 12 days when their ovaries were considered to contain developing follicles on rectal palpation. In the research mares, the size of the largest follicle in the ovaries during progesterone treatment did not differ from pre-treatment or control values; nevertheless, the 5 mares ovulated synchronously 11.2 +/- 0.4 (s.d.) days after progesterone withdrawal. Ovulation was less synchronized after progesterone withdrawal in the commercial mares (23 +/- 9.5 [s.d.] days) and in neither group did progesterone treatment advance the mean date of first ovulation compared with the untreated control mares. Mean plasma LH levels were not affected and mean FSH levels were slightly, but not significantly, elevated during progesterone administration in the research mares. In contrast, mean FSH levels fell steadily with time in both the progesterone treated and the control commercial mares. LH and FSH pulse frequencies in treated and control research mares differed only after progesterone withdrawal, when pulse frequency was higher in controls. We conclude that exogenous progesterone has no consistent effect on follicular development and gonadotrophin secretion patterns in transition phase mares, and in the present study it did not advance the mean date of the first ovulation of the breeding season compared with untreated control mares.