The inhibitory effects of ATP and adenosine on the nerve-mediated contractile responses of isolated sartorius muscle of the frog, Rana ridibunda, evoked by electrical field stimulation (EFS) were studied using pharmacological organ-bath technique. The effects of hydrocortisone applied in vitro and in vivo on contractility of sartorius muscle were also examined. ATP (100 microM) significantly reduced the amplitude of contraction to EFS of sartorius muscle, while pyridoxalphosphate-6-azonphenyl-2',4'-disulfonic acid (PPADS; 10 microM), a P2 receptor antagonist, abolished inhibitory effect of ATP. A similar inhibitory effect of adenosine (100 microM) was fully antagonized by 8-(p-sulfophenyl)-theophylline (8-SPT, 100 microM), a P1 receptor antagonist. Incubation of the tissue with hydrocortisone (10 microM) caused a slight, but significant, decrease of muscle contractions. After incubation of muscle preparations with both hydrocortisone and ATP, no inhibition of muscle contractility was registered. A single injection of hydrocortisone (100 mg/kg) 12 h prior to experiments to frogs did not significantly change the nerve-mediated contractility of isolated sartorius muscle; however, it abolished the inhibitory action of ATP without changing inhibitory activity of adenosine. After treatment of frogs with hydrocortisone for 14 days (100 mg/kg/day), both ATP and adenosine retained their inhibitory action on EFS-induced contractions of the muscle, and their effects were antagonized by PPADS and 8-SPT, respectively. It is concluded that hydrocortisone has antagonistic actions against the inhibitory effects of ATP at the frog neuromuscular junction, although this effect is lost following long-term treatment with hydrocortisone.