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The clinical observation that diabetic patients who exercise recquire less insulin therapy than those who do not, led to interest in the behaviour of circulating insulin concentrations during exercise. An increase in insulin concentration should result in increased glucose transport, and thus a decrease in blood glucose concentration. Studies of diabetic patients have shown (Sanders et al l961) that blood glucose concentration does, in fact, decrease as a result of exercise. However, since the same effect can be elicited in pancreatectomized animals (Ingle et al 1950, Goldstein et al 1953, Dulin and Clark 1961), it seems impossible that an increase in plasma insulin concentrations could be the only cause of the increased glucose transport. Thus a decrease in plasma insulin concentrations was also a possibility, and because of the necessity of preserving the blood and liver glucose for utilization by the central nervous system, this latter possibility seemed logical. A few studies of the effect of various types of exercise on plasma immunoreactive insulin (IRI) concentrations have demonstrated a decrease (Cochrane et al 1966, Rasio et al 1966, Hunter and Sukkar 1968). A study of insulin secretion in both normal and diabetic patients showed no increase as a result of exercise (Nikkila et al 1969). Studies of exercising animals have shown that insulin concentrations could be decreased during exercise even when glucose was infused (Issekutz et al 1967, Wright and Malaisse 1968).