It has been reported that the level of serum HDL-cholesterol in diabetics is lower than in normal subjects and that of insulin-treated subjects is higher than before treatment with insulin. The increase in serum HDL-cholesterol in insulin-treated diabetics is thought to be due to an increse in lipoprotein lipase activity. There have been few reports discussing the relationship between the level of serum HDL-cholesterol and tha t of serm insulin.In the present study, the relationship between these two parameters was therefore investigated in rats treated with a low dose of streptozotocin. Such rats demonstrated hyperinsulinemia probably due to insulin-producing tumor. There was, however, no relationship between the two above parameters. Hyperinsulinemia itself is not thought to influence the level of serum HDL-cholesterol.