The incidence of side effects due to two dosage regimens of minocycline was examined over a 5-day period. A total of 60 normal women volunteers were randomly assigned in a double-blind manner to either a group who took 100 mg of minocycline twice a day or a group who took 75 mg of minocycline twice a day for 5 days. Both groups were comparable from the standpoints of age, size, race, and the use of oral contraception, nicotine, and ethanol. They were seen on a daily basis, and symptoms were evaluated by both volunteers (from diaries) and physicians. Minocycline serum concentrations were determined on blood samples taken 2 h after the a.m. dose. Volunteers taking 150 mg of minocycline per day had significantly lower serum antibiotic concentrations than those taking 200 mg per day. However, both low- and high-dose groups exhibited similar incidence and prevalence of recorded symptoms, with the single exception of nausea, where the low-dose group had fewer symptoms than the high-dose group (P = 0.035). Symptomatic volunteers did not have higher serum concentrations of minocycline than their asymptomatic counterparts. When either weight or surface area was examined with antibiotic serum concentration there was a significant inverse correlation between the two on day 2 for both groups and also on day 4 for the low-dose group. It is concluded that, in women, a dose of 150 mg of minocycline per day is associated with the same degree of side effects as a dose of 200 mg per day.