AIM Action of adenosine triphosphate(ATP)on longitudinal muscle strips of the rat distal colon has been reported, however, that of the rat proximal colon remains to be clarified. In this study we investigated the effects of ATP on longitudinal muscle strips isolated from the rat proximal colon and the receptors involved in the effects. METHODSIsometric relaxant and contractile responses to ATP (0.1 μmol · L- 1 - 1 mmol· L- 1 ) and adenosine (1 - 100 μmol· L-1) in longitudinal muscle strips of the rat proximal colon were observed. RESULTS ATP (0.1 μmol· L- 1 - 1 mmol· L- 1 ) produced a complicated response including an inhibition of rhythmic contraction and a weakly transient decrease in basic tone (0.05-0.08 g) followed by a concentration-dependent contraction (0.04- 0.44 g) in longitudinal muscle strips of the rat proximal colon at resting tension. Tetrodotoxin (0.1 μmol·L-1) did not influence the responses to ATP.Adenosine (1 - 100 μmol· L-1) did not produce an obvious contractile response in the preparation at resting tension. Concentration-dependent relaxant responses to ATP ( 1 μmol· L- 1 - 1 mmol· L- 1 ) in the preparation precon tracted with 5-hydroxytryptamine or with acetylcholine were 23.2％ - 94.6％ or 24.8 ％ - 92.4％, however the relaxant responses to adenosine were much weaker than those to ATP. CONCLUSION ATP produces contractileresponsesmainly via purine and pyrimidine (P) 2receptors and relaxant responses partially via P1 receptors in longitudinal muscle strips of the rat proximal colon.