The daily administration of ergotamine (0·2 mg) or dihydroergotamine (0·2 mg) to rats from the 12th to 21st day of pregnancy caused considerable pup mortality. This appeared to be due to a toxic effect in utero rather than an inhibition of lactation. The daily administration of ergotamine (0·2 mg) to rats from 2nd to 13th day of lactation caused a significant reduction in growth-rate of the pups. There was also a significant reduction in food intake by the lactating rats. Daily administration of dihydroergotamine (0·2 mg) to rats over the same period had no effect on lactational performance. When studied by the paired-feeding technique, daily administration of ergotamine (0·4 mg) had no effect on lactational performance in the rat. These findings appear to contradict previous reports, and have been discussed in relation to them.