Pregnant mice were ovariectomized 3 days after mating and pregnancy was maintained by progesterone treatment. Different regimens of intraperitoneal injections of 3'-deoxyadenosine were applied 3 days after ovariectomy. Repetition of the injections at 3 h intervals induced a significant rate of implantation by delayed blastocysts. Other types of treatment with 3-deoxyadenosine as well as adenosine injections were found to be ineffective. Paradoxically, 3'-deoxyadenosine treatment had an adverse effect on the overall pregnancy rate found at autopsy.