The action of the adenyl compounds adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and adenosine was studied on the human blister base preparation. All 4 adenyl compounds produced pain which was slow in onset and not maintained. The threshold concentration for pain was of the order of 1-3 micron. The slopes of log concentration:pain intensity plots were relatively shallow and for moderate to severe pain a 100-fold increase of the threshold concentration was required. The adenyl compounds resembled 5-hydroxytryptamine and bradykinin with respect to onset and duration of action but were less potent. On the other hand, for threshold effects they were more potent than acetylcholine or potassium. Evidence was found for an interaction of adenyl compounds with 5-hydroxytryptamine but not with potassium, acetylcholine or bradykinin. Cyclic adenosine 3',5'-monophosphate or the chelation of extracellular calcium or magnesium were shown not to be involved in the algogenic action of adenyl compounds and the action of adenyl compounds on the rabbit isolated jejunum too was found to be unrelated to their algogenic action on the human blister base preparation.