The primary objective of this study was to determine whether bovine placental lactogen stimulated additional mammary growth as assessed by milk yield from a lactation induced by steroids. Pubertal, nonpregnant Holstein heifers (n = 23) were given daily subcutaneous injections of estradiol-17 beta (0.05 mg/kg) and progesterone (0.25 mg/kg) for 7 d to initiate mammary growth. Prolactin secretion was suppressed in all heifers via bromocriptine, which was administered until d 15. Heifers were treated with either placental lactogen (40 mg/d; n = 12) or water (control group; n = 11) for 18 d. Lactation was induced by daily injection of dexamethasone for 3 d and twice daily injections of recombinant bovine prolactin for 5 d starting on d 18. From 3 to 8 wk of lactation, milk yield of heifers treated with placental lactogen was numerically higher (22%) than the yield of control heifers, but the difference was not significant because of the high coefficient of variation. Daily injection of bovine somatotropin (d 57 to 66 of lactation) increased milk yield of both groups and stimulated a greater numerical increase in milk yield for heifers that were treated with placental lactogen. These results support the hypothesis that bovine placental lactogen is mammogenic and is one of the factors that regulates mammary growth during pregnancy.