Ovarian function was compared between nulliparous heifers (n=29; 10 to 16 mo old) and lactating Holstein cows (n=31; 55.9±3.5 d postpartum). Follicular dynamics, corpus luteum growth, and regression, and serum steroid concentrations were evaluated through ultrasonography and daily blood sampling. Most heifers (27 of 29) but only 14 of 31 cows had typical spontaneous estrous cycles after cycles were initiated. Twelve cows had atypical cycles, and 5 became anovulatory during the study. The 12 cows with atypical estrous cycles had low serum estradiol after luteolysis and failed to ovulate the dominant follicle present at luteolysis. Heifers and cows with typical cycles were compared directly. Interovulatory intervals were similar between heifers (22.0±0.4 d) and cows (22.9±0.7 d). Those animals had estrous cycles with either 2 (15 heifers; 11 cows), 3 (9 heifers; 2 cows), or 4 follicular waves (3 heifers; 1 cow). Cows ovulated later after luteolysis than heifers (5.2±0.2 vs. 4.6±0.1 d, respectively), and had more multiple ovulations (17.9 vs. 1.9%). Maximal serum estradiol concentration preceding ovulation was lower in cows than in heifers (7.9±0.8 vs. 11.3±0.6 pg/mL) even though ovulatory follicles were larger in cows (16.8±0.5 vs. 14.9±0.2mm). Similarly, maximal serum progesterone concentration was lower for cows (5.6±0.5 vs. 7.3±0.4 ng/mL), whereas maximal volume of luteal tissue was larger for cows than heifers (11,120±678 vs. 7303±308mm3). Thus, higher incidence of reproductive anomalies in lactating cows, such as low conception rate, ovulation failure, delayed ovulation, and multiple ovulations, may be due to lower circulating steroid concentrations in spite of larger ovulatory follicles and luteal structures.