STUDIES ON THE MECHANISM OF NIDATION

Summary. Impregnated rats were given a subcutaneous injection of ergocornine methanesulphonate between 0·15 and 0·5 mg/rat on the 4th day of gestation. The earliest response studied was the appearance of the oestrous vaginal smear, 2 to 3 days after drug injection. Gestation was terminated in three-quarters of the rats exhibiting vaginal oestrus. The ED50 for induction of oestrus and for termination of pregnancy was approximately 0·3 mg. Following the ergocornine-induced oestrus, the minority of rats exhibited normal oestrous cycles. Most rats had a spontaneous pseudopregnancy. During this pseudopregnancy, deciduomata could be elicited by `physiological' means, i.e. systemic histamine release. The weight of these deciduomata was, however, somewhat depressed. It is pointed out that a single injection of the drug elicits a complex series of endocrine sequelae.