Isolated rat sciatic nerves were used to study the interaction between 2-chloroprocaine (2-CP) and bupivacaine (BP). Five nerves studied as controls were treated with 5 × 10–4m BP and the amplitude of the compound action potential (CAP) evoked by suprathreshold stimulation was measured. This concentration of BP completely blocked nerve conduction; but, following washout with normal Krebs–Ringer solution, the CAP amplitude recovered to 50% of initial values in 50 (±4) min with a rate of recovery of 1.7 (±0.6) %/min. In another series of experiments, five nerves were blocked first with 5 × 10–4m 2-CP, allowed to fully recover, and then were blocked with BP under the same conditions as the controls. Under these conditions, the half time for the recovery of CAP amplitude following BP was shortened to 25 (±5) min, with a rate of recovery of 2.8 (±0.3) %/min. When five nerves were exposed to a 5 × 10–4m solution of a 2-CP metabolite, 4-amino-2-chlorobenzoic acid, no nerve blockade was produced. When these nerves subsequently were blocked with BP, recovery to 50% of initial values occurred in 22 (±5) min, with a rate of recovery of 2.0 (±0.2) %/min. Although pretreatment with either 2-CP or 4-amino-2-chlorobenzoic acid significantly shortened the duration of BP-induced nerve blockade, neither drug had a significant effect on the rate of recovery once the CAP amplitude returned to measurable values.These results suggest that the metabolite of 2-CP, 4-amino-2-chlorobenzoic acid, remains in the nerve following recovery from neural blockade and interferes with the subsequent action of BP upon this nerve.