Massed presentation of unsignaled shock results in less conditional freezing to contextual cues than do distributed presentations. Consistent with an account of the learning deficit based on the perceptual-defensive-recuperative theory, the massed-shock deficit was attenuated by preexposure to shock or the conditioning context. This formulation was also successfully applied to the deficit in conditioning that occurs when a single shock is given immediately after placement in a context. Opponent-process theory was not supported by 2 findings: (a) The deficit was neither enhanced by shock preexposure nor reduced by an opioid antagonist, and (b) unconditional reactions were greater with massed shock. Inconsistent with the suggestion that the effect is a performance artifact specific to freezing, the massed-shock deficit was apparent for a 2nd measure of conditioning.