1. The effects of the (+)- and the (-)-isomer of the phenylalkylamine derivative D888 (desmethoxyverapamil or devapamil) on isometric force and slow Ca2+ inward current were investigated in short toe muscle fibres of the frog (Rana temporaria). The experiments were performed under voltage-clamp conditions with two flexible internal glass microelectrodes at 10 degrees C in a TEA sulphate solution containing approximately 4 mM-free Ca2+. 2. In the presence of 0.05-5 microM-(-)-D888 a normal phasic contracture could be induced by a depolarizing voltage step. When depolarization was maintained for some minutes the force-controlling system turned into a stabilized inactivated state (paralysis) from which it recovered upon repolarization within minutes instead of seconds. With the (+)-isomer (0.5-20 microM), a similarly retarded restoration was observed. However, it proved to be less effective than the (-)-isomer. 3. D888 caused a shift to more negative potentials of the S-shaped curve, which describes the voltage dependence of force restoration in the steady state (restoration time 15 min). The potential of half-maximum restoration in the absence of the drug (V = -35.8 mV) changed as follows. (-)-D888: -56 mV (0.05 microM), -69 mV (0.2 microM), -77.5 mV (0.5 microM), and -82 mV (5 microM); (+)-D888: -55.8 mV (0.5 microM), -76.5 mV (5 microM), and -85 mV (20 microM). 4. On the assumption that D888 binds only to the inactivated form of the voltage sensor of force control in the T-tubular membrane (modulated receptor hypothesis) the data presented in paragraph 3 allowed an estimation of the drug-receptor dissociation constants. The KD values ascertained in this way, 1.71 nM for the (-)-isomer and 12.9 nM for the (+)-isomer, are in fair agreement with those obtained from [3H]D888 binding studies by other authors. 5. A comparison between equal concentrations of the two isomers regarding their effect on the speed of restoration and the time needed to transform the sensor into the paralysed state suggests that the differences in the dissociation constants are mainly due to a greater dissociation rate of the (+)-isomer from the sensor. 6. The restoration of the Ca2+ channel was retarded by D888 to a similar extent as that of the voltage sensor. This parallel action on both systems indicates structural similarities between the voltage sensor and the Ca2+ channel. 7. It is concluded that D888 'stabilizes' the inactivated state of the voltage sensor and the Ca2+ channel in a way similar to D600, but with a higher potency. Both isomers of D888 showed an antagonistic action and differed only in their potency.