The impact of a copayment increase on the utilization of psychiatric services in a prepaid group practice program is examined. Data are presented on utilization at the Columbia Medical Plan (Columbia, Maryland) two years before and two years after the copayment increase. There was a highly transient response to the increse in the copayment for psychiatric care. The year of the copayment increase was characterized by a small decline in the proportion of enrollees using psychiatric care, and a slight decrease in the utilization rate. These declines were short-lived, and utilization returned to previous levels one year after the copayment increase.