1. Interactions between the effects of adenosine or 2-chloro-adenosine (CADO) and the effects of substances that interfere with the phosphoinositides/protein kinase C transducing system or with the adenylate cyclase transducing system, on endplate potentials (e.p.ps), were investigated. The preparation used was the innervated sartorius muscle of the frog in which twitches had been prevented with high magnesium concentrations. 2. The activator of protein kinase C, 4 beta-phorbol-12,13-diacetate (PDAc), reversibly increased the amplitude and the quantal content of e.p.ps and attenuated the inhibitory effects of adenosine and CADO on e.p.p. amplitude. The affinity of the adenosine receptor antagonist, 8-phenyltheophylline, was not modified by PDAc. 3. The phorbol ester 4 alpha-phorbol-12,13-didecanoate, which does not activate protein kinase C, did not modify either e.p.p amplitude or the inhibitory effect of adenosine on e.p.ps. 4. The inhibitor of protein kinase C, polymyxin B, reversibly decreased the amplitude and the quantal content of e.p.ps, prevented the enhancement caused by PDAc on e.p.p. amplitude, but did not modify the inhibitory effect of adenosine on e.p.ps. H-7, another inhibitor of protein kinases, also decreased e.p.p. amplitude but did not modify the effect of PDAc on the amplitude of e.p.ps. 5. Lithium chloride, which alters phosphoinositide signal transduction by inhibiting the breakdown of inositol phosphates, reversibly increased the amplitude and the quantal content of the e.p.ps. In the presence of adenosine or CADO the effect of lithium on e.p.p. amplitude was markedly attenuated. 6. The activator of adenylate cyclase, forskolin, reversibly increased the amplitude and the quantal content of the e.p.ps. 7. The results suggest that the phosphoinositides/protein kinase C transducing system, but not the adenylate cyclase transducing system, might be involved in the inhibitory effect of adenosine on neuromuscular transmission.