The effect of LCB29 was tested on twitch characteristics, tetanic tension, and K+ and voltage-clamp contractures of rat soleus muscle fibers. In concentrations ranging from 10(-6) to 5 x 10(-4) M, LCB29 simultaneously inhibited the twitch amplitude, the maximum rate of tension development, and the maximum rate of relaxation. In concentrations ranging from 10(-5) to 10(-4) M, tetanic tension (100 Hz, 1 s) was inhibited by the same amount. The effect of 5 x 10(-5) M LCB29 was studied on K+ contractures and contractures induced, under voltage-clamp conditions, by long-lasting depolarizations. Its effect was significantly stronger than those on twitch and tetanic tension. In addition, LCB29 had a dual effect on strength--duration curves for mechanical threshold. It increased both the rheobasic potential and the steepness of the curve. It is concluded that LCB29 exerts a direct myorelaxant effect on rat soleus muscle; two sites of action are probably involved.