An interrupted time‐series analysis of local directory‐assistance calls in the Cincinnati area from 1962 to 1976 revealed a significant reduction in the daily frequency of calls after charges were introduced in 1974. No reductions occurred in the daily frequency of long‐distance directory‐assistance calls, which remained free. The results attest to the efficacy of response‐cost procedures with large subject populations in the natural environment. The applicability of response‐cost procedures to social and business problems i