The liver arginase in the rat decreases following adrenalectomy (Fraenkel-Conrat, Simpson & Evans, 1943; Folley & Greenbaum, 1946; Kochakian & Vail, 1947), and in lactating rats the mammarygland arginase behaves similarly (Folley & Greenbaum, 1946). It is possible that the decline in lactation causedby adrenalectomy (see, e.g., Cowie& Folley, 1947 a) is connected with the decrease in mammary-gl...