The contribution of conditioned responses (CRs) to homeostasis may be seen by examining seemingly disparate phenomena of color vision (aftereffects and chromatic adaptation) and drug addiction (withdrawal symptoms and tolerance). Color aftereffects may be elicited by nonchromatic stimuli previously paired with color (the McCollough effect, [ME]). Similarly, pharmacological aftereffects may be elicited by nonpharmacological stimuli previously paired with a drug (withdrawal symptoms). The authors summarize evidence indicating that both the ME and withdrawal symptoms are CRs. The chromatic CR is expressed as chromatic adaptation in the presence of color, and the ME in the absence of color. The pharmacological CR is expressed as pharmacological adaptation (tolerance) in the presence of the drug, and withdrawal symptoms in the absence of the drug. Both drug withdrawal symptoms and the ME are manifestations of the contribution of conditioning to normal homeostatic regulation. The authors discuss the implications of this conclusion for understanding regulatory processes and the evolution of behavioral mechanisms.