Pregnant mice fed on a diet containing agroclavine (250 μg/day) during the 2 to 3 days before nidation failed to implant but returned to oestrus within a few days. Eight weeks of agroclavine treatment did not impair the subsequent fertility of virgin mice. Female mice fed on the agroclavine diet conceived normally, and alkaloid treatment during the first 2 days of pregnancy or after implantation did not affect the pregnancy. Agroclavine did not affect pregnancy when injected subcutaneously. Ergotoxine, ergosine and lysergic acid α-hydroxyethylamide were also found to interrupt pregnancy when administered in the diet.