The effect of the addition of phenoxybenzamine to a regular insulin preparation on the absorption of insulin and plasma glucose concentrations was investigated in five normal subjects. The addition of phenoxybenzamine (20 micrograms) to insulin (0.2 U/kg body wt) resulted in an acceleration of insulin absorption, with a higher and earlier peak insulin concentration at 45 min and higher insulin concentrations throughout the 180 min of investigation. With phenoxybenzamine, plasma glucose concentrations fell more rapidly; nadirs were lower and more rapidly achieved. Two subjects experienced hypoglycemic symptoms when phenoxybenzamine was added to insulin. It is concluded that the addition of phenoxybenzamine to subcutaneously injected insulin increases the bioavailability of insulin for at least 3 h, which leads to a greater fall in plasma glucose concentrations.