Reproductive activities of the black-tailed prairie-dog have been compared on laboratory-confined and newly Oklahoma captured animals. Both males and females are reproductively inactive from June to October. Males exhibit the beginning of a new reproductive cycle in October with spermatozoa present by January. Females show renewed activity in December; cornifies vaginal smears and breeding occurs late in January or early February. Pregnancies are largely terminated in March. Lap-arotomies on 13 pregnant females gave an avg. litter size of 4.6. Evidence exists for resorption of embryos in utero, as well as a considerable loss of young after parturition. Young are helpless at birth and remain dependent upon the mother for approx. 7-8 weeks. On the range they appear above ground at an estimated age of 8 weeks.