The plasma insulin response to glucose and arginine was increased in intact female rats implanted with progesterone. This change was not mediated by increased energy intake since the insulin response to glucose was similarly enhanced in progesterone-treated rats pair-fed with controls. In contrast, similar levels of pancreatic glucagon were observed in the plasma of both control and experimental rats during arginine stimulation. Thus the molar ratio of insulin to glucagon may be increased in the plasma of progesterone-treated rats following a mixed meal. This change may represent a physiological adaption to a demonstrable resistance to the peripheral actions of insulin in non-adipose soft tissue. The hyperglycaemic and lipolytic actions of exogenous glucagon were also impaired in progesterone-treated rats. It is suggested that an increase in the molar ratio of insulin to glucagon will favour hepatic lipogenesis and the peripheral deposition of fat.