The effects of 2,3-butanedione monoxime (BDM, 0.5-20 mM) on Ca(2+)-activated force in skinned muscle fibres, and force and Ca2+ responses in aequorin-injected intact fibres from the iliofibularis muscle of the cane toad Bufo marinus were investigated. Peak twitch force responses progressively decreased to 3% of the control response with increasing [BDM] up to 20 mM. Peak twitch aequorin light responses decreased to 65% of the control response in 10 mM BDM, but increased again to control values in 20 mM BDM. The duration of the twitch aequorin light response increased by up to 60% above 5 mM BDM. Tetanic (170 Hz) force and aequorin light responses reversibly decreased in a dose-dependent fashion to about 50% of the control response in 10 mM BDM. Failure of tetanic (170 Hz) stimulation was observed in the presence of 20 mM BDM. Intracellular [Ca2+] could be modified by changing the frequency of tetanic stimulation in the presence of BDM, permitting a study of the dependence of isometric force on intracellular [Ca2+] at different concentrations of BDM. In 10 mM BDM, the rate of force development in intact fibres was slower by a factor of two at saturating [Ca2+], and was up to one order of magnitude slower at non-saturating [Ca2+], when compared with control responses. At a similar intracellular [Ca2+] steady-state isometric force was reduced to about 85 and 50% of the control responses in 2 and 10 mM BDM, respectively. The effect of BDM on maximum Ca2+-activated force in skinned fibres paralleled the decrease in tetanic (170 Hz) force observed in intact fibres. The rate of force development in skinned fibres decreased with an increase in [BDM] at constant [Ca2+], and the sensitivity of the contractile apparatus to Ca2+ was shifted to a higher [Ca2+] by BDM. The results suggest that BDM reduces contractility in cane toad iliofibularis muscle by direct inhibition of the contractile apparatus, and reduction of the release of activator Ca2+ from the sarcoplasmic reticulum. Furthermore, BDM may be a useful tool to help study the relationship between force and [Ca2+] in intact muscle fibres.