1. Serum and lipoprotein cholesterol and triglycerides were measured before, during and after the administration of glutethimide (500 mg daily) for 21 days to six healthy volunteer subjects. 2. Evidence of enzyme induction was provided by significant rises in d-glucaric acid excretion and antipyrine clearance. 3. Concentrations of total serum cholesterol, very-low-density-lipoprotein-, low-density-lipoprotein-and high-density-lipoprotein-cholesterol rose significantly during treatment. 4. The time course of these changes was delayed in comparison with the rise and fall in d-glucaric acid excretion. 5. There was no change in the triglyceride content of either whole serum or lipoprotein fractions at any time during the trial. 6. The study provides further evidence that enzyme-inducing agents cause a rise in certain lipid concentrations.