Female mice fed during the latter part of pregnancy on a diet containing sclerotia of Claviceps fusiformis at 2 to 3% failed to raise their litters. The active principle of these sclerotia in inducing the pup mortality has been shown to be the alkaloid agroclavine which was found to inhibit normal mammary hypertrophy when administered in the food at 5 to 7 mg%. Where the alkaloid treatment continued after parturition the litters died of malnutrition. When the alkaloidal diet was discontinued on the day before parturition there was a rapid recovery from the effect of agroclavine resulting in a normal pup growth rate. Agroclavine was also found to suppress well established lactation. This inhibition of mammary hypertrophy in mice by agroclavine administered orally at a non-toxic dosage is a new pharmacological action for the clavine alkaloids.