SINCE Kochakian and Murlin (1935, 1936, 1937) performed their fundamental experiments on castrate dogs it has been well established that administration of androgenic substances causes a fall in the nonprotein nitrogen level (NPN) in blood and at the same time lowers the nitrogen excretion in the urine. It is well known also that insulin has a very similar effect when injected into the diabetic organism (Falkenhausen, 1925, ChaikofT and Forker, 1950)—in other words, insulin and the male sex hormone may, under certain circumstances, exert a similar protein anabolic effect. The interrelationship between testosterone propionate and insulin has therefore been investigated and as a first step the behavior of the NPN and blood sugar levels of normal and depancreatized dogs receiving testosterone has been studied. EXPERIMENT This study was carried out on eight well trained female dogs of a weight between 20–25 Kg. Three of them were surgically depancreatized and a standard diet was fed twice a day.