Forty-four women with absent ovaries who were referred to our centre for oocyte donation were treated with oestradiol valerate and natural progesterone. Serum concentrations and endometrial changes were evaluated on day 21 of stimulated luteal phases after the daily administration of 100 mg of natural progesterone in oil injected intramuscularly or after the ingestion of 300 mg of micronized progesterone or after vaginal administration of 300 mg or 600 mg micronized progesterone, respectively. Endometrial tissue did not show evidence of an adequate secretory pattern after orally administered progesterone. However, after intramuscular injection of 100 mg of natural progesterone and after the vaginal administration of 300 to 600 mg of micronized progesterone, the histologic and electron microscopic pictures were similar in all treatment groups and were close to the expected day 21 of the cycle. Serum levels of progesterone after intramuscular injection were five times higher than after vaginal administration.