Both placenta and prolactin are required for maintenance of gestation in rabbit. This study was undertaken to characterize the influence of placenta on uterus and mammary gland using the virgin pseudopregnant rabbit as a model system. Homogenates of 20-day rabbit placenta were found to stimulate increased growth of uterine and mammary tissue in pseudopregnant rabbits. Essentially, all of this uterine growth occurred in endometrium and represented growth of both stromal and epithelial cell types. After fractionation by successive extraction of rabbit placenta with acetone and bicarbonate, both extracts were found to stimulate uterine and mammary growth in pseudo-pregnant rabbits when measured on day 20 of pseudopregnancy after 13 days of treatment at extract doses equivalent to that found in 0.1 gram of placenta. Approximately 75% of this stimulatory activity is found in the acetone extract and the remainder appears in the aqueous extract. The extracts stimulate a maximum 90% increase in uterine growth and 400% increase in mammary pad area. Both activities exhibit logarithmic response characteristics, yet significant changes in serum 17 beta estradiol, progesterone, 20 alpha dihydroxyprogesterone and prolactin did not occur. These observations suggest the presence of an unidentified factor in rabbit placenta which has a non-ovarian mediated stimulatory activity on endometrial and mammary growth.