Deoxycytidine antagonizes the analgesic action of morphine in the rat, morphine-induced respiratory depression in the rabbit and mitigates withdrawal in the dependent mouse. Administration of deoxycytidine does not precipitate the abstinence syndrome in dependent mice, a property shared by certain other endogenous morphine antagonsits. Cytidine and thymidine closely related in structure to deoxycytidine, are interchangeable with saline as controls, indicating a high degree of specificity in the receptor involved in the "action-effects" sequence. The requirement for deoxycytidine in countering the effects of morphine parallels the requirement for certain other DNA pyrimidines in the antagonism of barbiturate and alcohol. The rapidity of antogonist action suggests that such a DNA species would presumably be localized in the synaptosome, at outer membrane or the synaptic junction.