The effects of age and of gender on the plasma profiles of HMG-CoA reductase inhibitors following separate once-a-day dosage regimens (17 days) of lovastatin (80 mg/day) and simvastatin (40 mg/day) were studied in hypercholesterolemic patients. In general, plasma concentrations of active and total HMG-CoA reductase inhibitors were higher in elderly individuals (age, 70 to 78 years) and in females for both drugs. However, the Tmax of these inhibitors was not significantly affected by either age or gender. Following the last dose of lovastatin, the mean steady-stage plasma concentrations of total and active HMG-CoA reductase inhibitors were 30-60% higher in the elderly than in young individuals (age, 19 to 30 years). Also, the mean plasma concentrations were 20-50% higher in female than in male patients. Similarly, following the last dose of simvastatin, the mean plasma concentrations of HMG-CoA reductase inhibitors were 40-60% higher in the elderly than in young patients and were 20-50% higher in female than in male patients. These age- and gender-related differences do not appear to be large enough to warrant modification of dosage regimens, because plasma concentrations of these inhibitors are not necessarily indicative of efficacy and the therapeutic windows for lovastatin and simvastatin are broad.