Traditional two-process theory predicts that a conditioned stimulus (CS) paired with shock offset on Pavlovian trials will inhibit operant avoidance responding. Two explanations of the inhibitory mechanism involved were compared: contemporaneous pairing of CS with a hedonic relief reaction versus the predictive, discriminative relationship of CS to the non-shock interval. The pattern of avoidance inhibition associated with cessation CSs paired with electric shocks of constant duration was expected to be different from the pattern accompanying cessation CSs paired with shocks of variable duration. Mean rates of responding by the two groups were compared by analysis of covariance using baseline as the covariate. Neither CS displayed any reliably observable effects on avoidance rates. Possible procedural flaws and compatible improvements are discussed.