In the present study a very simple test has been developed to demonstrate the time of onset of lactation in pregnant rats. Pro-oestrous female albino rats were placed with a male rat overnight. The next morning, if spermatozoa were found in the vaginal smear, was counted as the first day of pregnancy. In our colony, rats usually deliver on day 22. Starting on day 20 of pregnancy the rats were tested as follows: under ether anaesthesia, 50 m-u. oxytocin were given i.p. and a nipple was cut superficially. If the mammary gland is ready for lactation milk will flow out 1 or 2 min. after oxytocin administration. The whole procedure takes less than 5 min. and the rat recovers quickly. Using this test 47 pregnant rats have been studied. The time when the first foetus was delivered was taken as the time of parturition. In none of the rats used