Abstract The objective was to determine the effect of biostimulation (bull-exposure) on uterine involution (UI), plasma progesterone concentration (P 4), size of largest follicle (LF), number of follicles larger than 5 mm ( F≥5), presence of fluid in uterine lumen (PF), presence of luteal tissue (LT), and length of the first estrous cycle postpartum (LEC). Ninety Angus cows with calves were allocated by parity and body frame into three groups (30 per group) 1 week postpartum. Two groups were exposed to bulls (BE) and one non-exposed group (NE) served as a control. Data were collected during weekly sessions of palpation per rectum, ultrasonography and bleeding on a subgroup of 30 cows (10 cows per group) for 6 weeks, and permanent surveillance of estrus with HeatWatch ® on all 90 cows. There were no significant differences between BE and NE cows for UI (17.1±1.1 days versus 20.1±1.6 days), LF (9.5±1.7 mm versus 11.0±2.4 mm), F≥5 (1.20±0.3 versus 1.47±0.09), and PF. However, LT was detected in more BE than NE cows (13 versus 2; P<0.001). Overall differences in P 4 were found between BE and NE cows with detected LT (2.00±0.3 ng/ml versus 1.05±0.4 ng/ml, respectively; P<0.05). More BE cows resumed reproductive cyclicity with estrous cycles normal in length compared with NE cows (16/30, 53%; 16/30, 53%; and 8/30, 26.6%, for the two BE groups and the NE group, respectively; P<0.01). In conclusion, BE hastened luteal function but did not affect uterine involution.