Treatment of superovulated cows with bovine somatotropin (bST) at the time of insemination has been reported to decrease the number of unfertilized oocytes, while increasing the development rate and morphological quality of embryos (Thatcher WW et al., 2001 Theriogenology 55, 75–89). These findings suggest that bST treatment might be used to improve pregnancy rates in inseminated cattle. The objective of this study was to further investigate the effects of bST (Posilac; Monsanto Co., St. Louis, MO, USA) treatment at or after insemination on subsequent pregnancy rate in diary heifers. Crossbred dairy heifers (n = 73) between 14 to 15 months of age and averaging 331 ± 3.6 kg, were used for the study. Estrus was induced by a single or repeated (at 14-day intervals) injections of 25 mg of PGF_2α. Heifers were observed at least twice daily and artificially inseminated about 12 h after detection of estrus. At estrus, heifers were randomly assigned across treatments. Treatments were bST (Posilac) injection (500 mg, s.c.) at the time of insemination (Day 0), on Day 14, or both at insemination and on Day 14. Untreated animals served as controls. On Day 45 after estrus, ultrasonography was used to determine pregnancy status and measure fetal-crown rump length. Chi-square analysis was used to evaluate the effect of treatment on pregnancy rate. Analysis of variance was used to compare fetal crown-rump length among treatments. Results are presented in the table below. Treatment of heifers with bST at insemination had no effect (P = 0.306) on pregnancy rate when compared with the control group. However, bST treatment on Day 14, or on both Days 0 and 14, reduced (P = 0.009)pregnancy rate, when compared with the control and Day 0 bST treatment groups. Pregnancy rate was similar (P = 0.729) for heifers receiving bST on Day 14 v. Days 0 and 14. Fetal growth, as measured by crown-rump length, was unaffected by treatment (P = 0.603). Results indicate the timing of bST treatment after the onset of estrus can influence the subsequent pregnancy rate in dairy heifers.