To mimic the abnormal patterns of follicular growth in the early post-partum period, heifers were injected with 15 mg oestradiol and 37•5 mg progesterone every 12 h for 7 days starting on day 15 of the oestrous cycle. Control and treated animals (no. = 8 per group) were monitored by daily ultrasonography and plasma hormone measurements. Five treated animals had quiescent ovaries for 11 days during which period oestradiol concentrations exceeded 30 pg/ml. After this, the first post-treatment follicular wave produced a normal ovulatory follicle. The other three heifers had quiescent ovaries for 17 days during exposure to high oestradiol values but this was followed by the formation of persistent follicles. The prolonged exposure to oestradiol probably disrupted normal LH surges in the latter animals. Growth rates and internal diameters for normal, ovulatory and prolonged persistent follicles were similar for the first 6 days after emergence, but the persistent follicles grew larger (P < 0·05) than normal follicles over the next 20 days. Luteinization of a subsequent persistent follicle was observed in one heifer. For 2 to 3 days prior to follicular growth, FSH concentrations were similar in controls, and heifers with ovulatory or persistent follicles (P > 0·05). When follicles first emerged and for the next 4 days, oestradiol concentrations were greater in follicles which later persisted (P < 0·05). For the second half of the life-span of persistent follicles, oestradiol concentrations were basal, thus the structure remained for many days after endocrine function had declined. These observations allow new interpretation of previously confusing findings in subfertile cattle presented to veterinarians.