Summary. Single 100‐mg doses of progesterone were given orally and as intramuscular injections t o four women during the follicular phase of the menstrual cycle. After oral administration serum levels of progesterone increased rapidly to reach luteal phase values (mean maximum level 55.6 nm) within 1–4 h and were still elevated after 12 h. The serum concentrations of 20α‐hydroxy‐4‐pregnen‐3‐one showed a similar pattern while there were only minor transient changes in 17α‐hydroxypro‐gesterone concentrations. The serum levels of cortisol and 4‐androstene‐3, 17‐dione were unaffected. In comparison, after intramuscular administration values two to three times higher than by the oral route were achieved. A significant increase in serum deoxycorticosterone was recorded in all women. The mean ratio between the change in deoxy‐corticosterone and progesterone was increased after oral administration. Oral treatment with natural progesterone may develop into a n attractive alternative to synthetic progestogens but the conversion of progesterone into a potent mineralocorticoid may be a potential disadvantage.