Increased insulin sensitivity of vitamin B6-deficient rat adipose tissue may be related in part to altered GH metabolism. The metabolism of glucose-U-14C in muscle and adipose tissue was determined in vivo as a function of insulin, GH and combined insulin-GH treatments. Vitamin B6-deficient rats were more sensitive to insulin and acute GH treatments, which exerted an insulin-like effect, than were ad libitum or pair-fed controls. Chronic GH treatment had an insulin-like effect on muscle and potentiated the action of insulin, while it inhibited the action of insulin in adipose tissue. The degree of response to chronic GH treatment was greatest in the vitamin B6-deficient rats. The data provide evidence that chronic GH treatment modified insulin sensitivity in vitamin B6-deficient rats.