We investigated the influence of the calcium-/calmodulin antagonist perhexiline on the calcium sensitivity of skinned muscle fibres from rabbit psoas muscle suspended in MgATP salt solution (pH 7.0; 20 degrees C). Perhexiline strongly sensitized the skinned fibres for calcium; thus the relationship between isometric force development and calcium ion concentration was shifted to the left in the presence of perhexiline. Similar results have been achieved with skinned fibres from pig cardiac (ventricular) muscle fibres. In skinned psoas we also determined the binding of Ca2+ to troponin-C by using the method developed by Güth and Potter (1987). Troponin-C labelled with DANZ (5-dimethyl-aminonaphthalene-2-sulfonyl aziridine) was incorporated into skinned fibres after previous extraction of troponin-C. The fluorescence signal (at 460 nm) as well as force increased when the calcium ion concentration rose to pCa 5.7. Addition of perhexiline increased this force and the fluorescence signal indicating that the increase in calcium responsiveness of the contractile apparatus was, at least partly, due to an increase in calcium occupancy of troponin-C.