Spontaneous contractions of liver fluke muscle were abolished in Ca²⁺-free saline and by 100 µM nifedipine and reduced by 5 mM cadmium chloride, suggesting that they are dependent on extracellular Ca²⁺. Caffeine (5 mM) significantly increased contraction amplitude and frequency. Ryanodine (100 µM) failed to block the caffeine response but significantly reduced spontaneous contraction frequency, suggesting that intracellular stores have a functional role. Cyclopiazonic acid (5 µM) had no effect on the caffeine response or spontaneous activity. 3-Isobutyl-1-methylxanthine (IBMX), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate significantly increased spontaneous contractions, which implies that cAMP has a regulatory function in motility. Caffeine, however, produced no measurable increase in cAMP. The caffeine effect was inhibited by cadmium chloride and nifedipine, whereas IBMX-induced increases in amplitude were reduced by cadmium chloride. Thus caffeine and cAMP appear capable of opening plasma membrane Ca²⁺ channels, but the involvement of cAMP in caffeine responses has not been proved.