The effects of chronic high sucrose feeding for 1 month on in vivo and in vitro insulin secretion and on in vivo insulin action were studied in normal male rats. As compared to the standard chow diet, the high sucrose diet induced excess in vivo insulin response to an intravenous glucose load; the high sucrose diet also slightly improved glucose tolerance, as demonstrated by significantly higher rate of glucose disappearance (p less than 0.02). The increased insulin secretion in response to glucose in vivo seems to be related to an hyper-reactivity of the pancreatic B cell to glucose, since it was still observed in vitro with the isolated perfused pancrease preparation. By contrast, B cells of sucrose-fed rats exhibited in vitro a normal response to arginine and a significantly lowered (p less than 0.05) response to acetylcholine. The insulin action in the sucrose-fed rats was quantified in vivo with the insulin-glucose clamp technique. The effects of different concentrations of insulin on glucose production and glucose utilization were studied in anaesthetized rats while in the postabsorptive state. The basal glucose utilization was found significantly higher (p less than 0.001) in sucrose-fed rats. During the clamp studies the glucose utilization induced by submaximal (400 microU/ml) or maximal (7500 microU/ml) insulin levels was significantly more important (p less than 0.02) in the sucrose-fed rats than in the chow-fed rats. This suggests that insulin-mediated glucose uptake is enhanced over a large range of plasma insulin levels in the sucrose-fed rats.(ABSTRACT TRUNCATED AT 250 WORDS)