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Five variables were manipulated in a pulse-rate conditioning study: CS duration, trace vs. delay procedures, order of presentation of CS, US, and CS-US trials, blocked vs. randomly mixed trials, and shock vs. noise US. One hundred ninety-two Ss were distributed across 12 conditions. The form of the cardiac response during extinction was a composite of the spontaneous response to the tone CS plus the conditional deceleration established by training. Conditioning efficiency was maximized by giving the training trials first, and by using a 5-sec. CS-US interval. An outline of the factors influencing cardiac conditioning is presented, and this outline is congruent with the theoretical analysis of classical conditioning proposed by Dykman and Grings.