The effect of L-N6-phenylisopropyladenosine (L-PIA) on the maximum tension developed (Td) and the maximum rate of rise of tension (dT/dt max) of the isolated hemidiaphragm of the rat was investigated during direct electrical stimulation. L-PIA itself (0.07-5.5 mumol l-1) did not produce significant changes in either Td, or dT/dt max. In the presence of a standard concentration of dipyridamole (26.4 mumol l-1), L-PIA produced a significant and concentration-dependent increase in both Td and dT/dt max. In the presence of a standard concentration of aminophylline (640 mumol l-1), L-PIA produced a small and insignificant, but concentration-dependent decrease of both Td and dT/dt max. In the presence of both dipyridamole (26.4 mumol l-1) and of aminophylline (640 mumol l-1), L-PIA (0.07-5.5 mumol l-1) produced a small and insignificant potentiation of the isometric contraction of the isolated hemidiaphragm. It is concluded that antagonistic action between L-PIA and aminophylline probably takes place at A1-receptors. The inhibitory action of L-PIA, observed after blockade of adenosine receptors by aminophylline, is presumably realized through some other mechanism(s) independent of adenosine receptor sites.