Antagonism of the antinociceptive action of morphine in mice by prior parenteral administration of cAMP has been confirmed using two additional analgesic testing procedures, the hot-plate test and the stretching test. The analgesia-antagonistic property, however, is not specific for cAMP because it also occurred after pretreatment with compounds which probably increased the concentration of adenosine or adenine in the body. Preliminary indications are that an unsubstituted purine ring is a requirement for the adenine derivative that is primarily responsible for the antagonism of the analgesic action of morphine.