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Rationale: To test the classical conditioning and expectancy theories of placebo effects. Objective: Two experiments investigated whether administration of caffeine-associated stimuli elicited conditioned arousal, and whether information that a drink contained or did not contain caffeine modulated arousal. Method: Experiment 1 (n=21) used a 2 Caffeine (0 and 2 mg/kg) × 2 Solution (Coffee, Juice) × 2 Information (Told caffeine, Told not-caffeine) within-subjects design. Experiment 2 (n=48) used a 2 Solution (Coffee, Orange juice) × 3 Information (Told caffeine, Told not-caffeine, No information) between-subjects design. Indexes of arousal were skin conductance responses and levels, startle eyeblink reflexes, cardiovascular measures, and the Bond and Lader 1974 mood scale. Results: Caffeine-associated stimuli increased alertness, contentedness and skin conductance levels, and information that the drink contained caffeine decreased calmness in Experiment 1. However, unexpected information about the caffeine content of the drink, and the order of the conditions, could have masked some effects of the experimental manipulations. Experiment 2 followed up this hypothesis. The results showed a conditioned increase in startle eyeblink reflexes, and that caffeine-associated stimuli together with information that the drink contained caffeine increased contentedness. Conclusions: Caffeine-associated stimuli increased arousal, and information about the content of the drink modulated arousal in the direction indicated by the information. Thus, both the classical conditioning and expectancy theories of placebo effects received support, and placebo effects were strongest when both conditioned responses and expectancy-based responses acted in the same direction. [ABSTRACT FROM AUTHOR]