In this study of the influence of copayment on drug utilization, we examined the prescription experience of 10 categories of drugs used by Medicaid beneficiaries in South Carolina over a four-year period. We observed an immediate and significant effect of copayment on the level of expenditures for all drug categories except analgesics and sedative/hypnotic drugs. The impact of copayment on the changes in the slopes of the series was not as dramatic. Only expenditures for cardiovascular, cholinergic, diuretic, and psychotherapeutic agents demonstrated a significant change in the long-term trend. We conclude that the imposition of a copayment for prescription services exerted a differential effect among therapeutic categories.