1. The blood concentrations of 3-hydroxybutyrate and alanine were measured before and after they were separately injected into obese patients who were taking a 1.68 MJ (400 kcal) diet and again when they had starved for 4 days and 28 days. 2. The disappearance of the injected metabolite was exponential and its half-life and volume of distribution were calculated. 3. The disappearance of ketones (both 3-hydroxybutyrate and acetoacetate) was raised in the patients when they were taking the low-calorie diet and increased a further sevenfold during starvation. This further increase was associated with an increase in the half-life of injected 3-hydroxybutyrate. 4. The pre-injection concentration of alanine almost halved during starvation and the half-life of injected alanine increased. 5. If the changes in half-life of exogenous 3-hydroxybutyrate and alanine apply to endogenous metabolites, then these results suggest that the increase in blood ketone concentration during starvation is at least partly due to a decreased clearance of ketones and the decrease in blood alanine concentrations is associated with a marked decrease in production rate of alanine.