The alkylating agent N-ethylameimide and the sulfhydryl group blocker p-chloromercuribenzoic acid (CPMA) inhibited in dose-dependent manner both basal activity of adenylyl cyclase (AC) and its activity stimulated by non-hormonal substances (forskolin, sodium fluoride, guanylilimidodiphosphate) in smooth muscles of the freshwater bivalve mollusk Anodonta cygnea. The double increase (from 30 to 60 min) in the time of preincubation of a sarcolemmal membrane fraction with ethylmaleimide and CPMA led to an essential increase in enzyme inhibition (especially for CPMA). 50 mM SH-containing reagent beta-mercaptoethanol (ME) partially restored the AC activity, inhibited by N-ethylmaleimide and CPMA, except when these two latter reagents were in high concentrations (1-10 and 0.5 mM, respectively). The data obtained point to the key role of cysteine SH-groups in regulation of the functional activity of proteins, components of the adenylyl cyclase system--AC and heterotrimeric G-proteins.