That the involuted thymus gland of pregnancy regenerates after delivery was first pointed out by Fulci [1913a] and confirmed by Bompiani [1914], Ohmura [1928], and Jolly & Lieure [1930]. Bompiani also reported that if delivery is followed by lactation the involution is maintained throughout the suckling period (though not in the guinea-pig [Jolly & Lieure, 1930], presumably owing to the irregular and poor nursing in this species). When the litter is removed regeneration of the gland sets in and is rapidly completed. Bompiani [1914] gave no information of the age, number, nor of the variations in body weight or thymus weight of the animals he used and the first object of the present work has been to confirm and extend Bompiani's results by collecting quantitative data on the rates of involution after parturition and weaning (21 days after delivery). The second object has been to investigate the possible influences