Two methods for synchronizing ovarian follicular development in both wood bison (Bison bison athabascae) and plains bison (Bison bison bison) were tested as part of a project to conserve wood bison through the application of advanced reproductive technologies. A secondary objective was to test the effect of a long-acting neurolept tranquilizer, pipothiazine palmitate, on ovarian function in bison. Female wood bison (4 years old; n = 14) and plains bison (2-8 years old, n = 10), previously conditioned to daily examination in a chute, were divided randomly into 3 groups in which 1) ovarian follicles ≥5 mm were ablated by ultrasound-guided transvaginal follicle aspiration, 2) 5 mg of estradiol-17β in canola oil was given i.m. or 3) no treatment was given (control). The experiment was conducted in 3 replicates so that each animal rotated through each of the groups. Half of the bison, blocked by subspecies and treatment group, were given a single dose (150 mg) of pipothiazine palmitate i.m. The ovaries were examined daily by transrectal ultrasonography beginning 4 days before treatment and continuing until the dominant follicle of a new wave reached a diameter of 10 mm. No effect of treatment with pipothiazine palmitate was detected for any end point. In addition, no differences were detected between wood and plains bison for any end point. Consequently the pipothiazine palmitate and subspecies treatment groups were collapsed for further analyses. The interval and variation in the interval to new follicular wave emergence were compared by ANOVA. Wave emergence was detected on Day 1.0 ± 0.2 (mean ± SEM; Day 0 = day of treatment) in the follicle ablation group, and was earlier (P < 0.05) than in both the estradiol (Day 3.3 ± 0.3) and control (Day 4.0 ± 0.4) groups. The interval to follicle wave emergence was least variable in the follicle ablation group (P < 0.05) and tended to be less variable in the estradiol group (P = 0.09) than in the control group (residuals, 0.1 ± 0.04, 1.0 ± 0.2, and 1.6 ± 0.3 days, respectively). Ovulations occurred subsequent to estradiol administration in 10 of 23 (43%) bison. In conclusion, ovarian follicular wave emergence can be synchronized in bison during the anovulatory season. Follicular ablation consistently shortened and decreased the variability in the interval to new wave emergence. The synchronizing effect of estradiol was confounded by the induction of ovulation. Progesterone will be added in future studies to control the ovulatory effect of estradiol. Pipothiazine palmitate had no discernable effect on ovarian function and maybe useful in reducing the effects of handling stress on untrained animals.