1. Porcine insulin (0·1 i.u./kg) was injected intravenously into diabetic and non-diabetic subjects.
2. In each group insulin produced a fall in blood glucose within 5 min which was exponential and which continued until approx. 99% of the insulin had left the plasma.
3. In the diabetic subjects both the insulin values and blood glucose fell more slowly than in the controls, when expressed as a proportion of the fasting value. However, both insulin and glucose continued to fall for a longer period, with the result that there was a total fall of blood glucose of 67% in the diabetics and 70% in the non-diabetics. Though in absolute terms considerably more glucose disappeared in diabetics than in the normal subjects, this could be related to the higher initial glucose concentrations in the former.
4. It is suggested that not only does insulin act very rapidly, but it is also rapidly inactivated.