In this work, we tested whether L-type Ca(2+ )channels are involved in the increase of caffeine-evoked tension in frog slow muscle fibers. Simultaneous net Ca(2+) fluxes and changes in muscle tension were measured in the presence of caffeine. Isometric tension was recorded by a mechanoelectrical transducer, and net fluxes of Ca(2+) were measured noninvasively using ion-selective vibrating microelectrodes. We show that the timing of changes in net fluxes and muscle tension coincided, suggesting interdependence of the two processes. The effects of Ca(2+)channel blockers (verapamil and gadolinium) were explored using 6 mM: caffeine; both significantly reduced the action of caffeine on tension and on calcium fluxes. Both caffeine-evoked Ca(2+) leak and muscle tension were reduced by 75% in the presence of 100 microM: GdCl(3), which also caused a 92% inhibition of net Ca(2+) fluxes in the steady-state condition. Application of 10 microM: verapamil to the bath led to 30% and 52% reductions in the Ca(2+)leak caused by the presence of caffeine for the peak and steady-state values of net Ca(2+) fluxes, respectively. Verapamil (10 microM): caused a 30% reduction in the maximum values of caffeine-evoked muscle tension. Gd(3+)was a more potent inhibitor than verapamil. In conclusion, L-type Ca(2+) channels appear to play the initial role of trigger in the rather complex mechanism of slow fiber contraction, the latter process being mediated by both positive Ca(2+)-induced Ca(2+ )release and negative (Ca(2+) removal from cytosol) feedback loops.