We examined the effects of ATP on intrinsic pump activity in lymph vessels isolated from rat. ATP caused significant dilation with a cessation of the lymphatic pump activity. Removal of the endothelium or pretreatment with N-nitro L-arginine methyl ester (L-NAME) significantly reduced the ATP-induced inhibitory responses of the lymphatic pump activity, while the reduction was not addressed completely with 10^-6 M ATP. L-arginine significantly restored the ATP-induced inhibitory responses in the presence of L-NAME. The ATP-induced inhibitory responses in the lymph vessels with endothelium were also significantly, but not completely, suppressed by pretreatment with glibenclamide. 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX, a selective adenosine A₁ receptor antagonist), but not suramine (P_2X and P_2Y receptors antagonist) or 3,7-dimethyl-1-proparglyxanthine (DMPX, a selective adenosine A₂ receptor antagonist), significantly decreased the ATP-induced inhibitory responses. ,-Methylene ATP (a selective P_2X and P_2Y agonist) produced no significant effect on the lymphatic pump activity. In some lymph vessels with endothelium (24 out of 30 preparations), adenosine also caused dose-dependent dilation with a cessation of the lymphatic pump activity. L-NAME reduced significantly the lower (3x10^-8~ 3x10^-7 M) concentrations of adenosine-induced inhibitory responses. Glibenclamide or DPCPX also suppressed significantly the adenosine-induced inhibitory responses. These findings suggest that ATP dilates and inhibits pump activity of isolated rat lymph vessels in endothelium-dependent and -independent manners. The ATP-mediated inhibitory responses may be, in part, related to production of endogenous NO, involvement of ATP-sensitive K⁺ channels or activation of adenosine A₁ receptors in the lymphatic smooth muscles and endothelium.