The non-cholinergic, non-adrenergic responses of the rainbow lizard (Agama agama Linn.) isolated rectum to transmural electrical stimulation have been examined in an effort to characterise them. In the presence of atropine (2 microM) and guanethidine (10 microM), tetanic transmural electrical stimulation (at 20 Hz) induced, in most of the preparations examined, a rapid initial relaxation (primary relaxation), which changed during the stimulation period into a contraction (primary contraction) followed by a rebound contraction (secondary contraction), which subsided at the end of the stimulus and was followed, in most preparations, by a relaxation (secondary relaxation) of variable extent and duration. Noradrenaline (1-10 microM) reduced or markedly inhibited the cholinergic muscle twitches, and decreased the various phases of the non-cholinergic, non-adrenergic muscle contractions, especially at low frequencies of stimulation (less than or equal to 10 Hz). At higher frequencies (greater than or equal to 20 Hz), ATP (5-10 microM) increased the primary relaxation, decreased the contractile phases, and inhibited post-tetanic inhibition induced by non-cholinergic, non-adrenergic nerve stimulation. Theophylline (0.1-1.0 microM) increased the primary and rebound contractions with no marked influence on the primary or secondary relaxation. Quinidine (5-50 microM) enhanced the primary relaxation, slightly inhibited the rebound contraction, but inhibited the primary contraction in a concentration-dependent manner. It is concluded that the P1 and P2 purinergic receptors proposed by Burnstock (10) are unlikely to mediate the non-cholinergic, non-adrenergic postsynaptic responses of the rainbow lizard isolated rectum.