Paw withdrawal data calculated as a percent of the maximal possible effect (%MPE) are plotted as a function of the change in EC₅₀ for the lidocaine-induced block of the A (A) and C (B) wave of the sciatic nerve compound action potential, relative to the mean EC₅₀ observed in nerves from naive rats. Data are from the rats treated with morphine at different doses and/or different amounts of time, illustrated in figures 2 and 3. The relationship between % MPE and EC₅₀ was linear and highly significant (P < 0.01) for both A and C waves.