4-14C-Lynestrenol** was administered either orally or by intravenous injection to women using Lyndiol®*** (5 mg lynestrenol + 150 μg mestranol). In order to study the effect of the pharmaceutical form in oral administration, the radioactive progestagen was given either as the pure compound in the form of a gelatin capsule or as a tablet containing all the ingredients usually present in Lyndiol®-tablets. The concentration of radioactivity in the blood and the excretion of radioactivity in the urine and milk of lactating women were then studied. The clearance rate of radioactivity from the blood was very low. A half-life in the order of 40 h was estimated for labelled »lynestrenol and metabolites«. Within 4–5 days after oral administration of a 14C-lynestrenol containing tablet, 54–65 % of the radioactivity was excreted in the urine, whereas much smaller amounts (in the order of 15 % of the administered dose) were excreted following oral administration of 14C-lynestrenol taken in a capsule. After intravenous administration 45–56 % of the administered radioactivity was recovered in the urine excreted within 4 days after injection. After oral administration of a 14C-lynestrenol containing tablet to 3 lactating women 0.022. 0.028 and 0.088 per cent of the administered radioactivity was excreted in the milk collected during the first 5 days after administration. The results indicate that resorption may be considerably influenced by the pharmaceutical form of the orally administered compound. For oral administration of 5 mg lynestrenol daily, as a Lyndiol®-tablet, it was calculated that in the order of 1 μg (0.02 % of the administered dose) of lynestrenol or its metabolites might be excreted per 100 ml of milk. These data are discussed with regard to the amount of steroids that might be transferred to infants during breast feeding when the mothers take oral contraceptives.