The effect of a 14 day-administration of butylbiguanide was investigated in a group of 10 obese patients with mild-to-moderate glucose intolerance. Glucose tolerance was significantly improved, while fasting blood glucose and plasma levels of free fatty acids, insulin and glucagon remained unchanged. The estimation of the amount of the oral glucose load oxidized into CO2 was performed by means of a recently described procedure using "naturally labelled 13C-glucose" as tracer. The curves depicting the oxidation of the exogenous glucose load were similar in shape and magnitude before and after administration of the biguanide; in the latter case, however, slightly higher rates of oxidation of exogenous glucose were recorded during the 2nd, 3rd and 4th hours of the test. These data do not provide evidence that the biguanide-induced improvement in glucose tolerance in patients with mild-to-moderate glucose intolerance is associated with any inhibiting or delaying effect of this type of drug on intestinal absorption (and subsequent oxidation) of the exogenous glucose load. On the contrary, a slight, but statistically significant, increase in the oxidation of exogenous glucose has been observed after butylbiguanide.