Ca-mediated action potentials (CaAP) from guinea pig papillary muscle have been reconstructed by means of a computer simulation. The reconstruction model is based as closely as possible on ionic currents which have been measured by voltage clamp methods. Three currents were formulated in terms of Hodgkin-Huxley type equations: a Ca inward current, a time-independent K outward current ("rectifier" current), and a time dependent slowly activated K outward current ("delayed rectifier" current). The maximum upstroke velocity of CaAP was compared with the Ca inward current. It turned out that the discrepancy between Vmax and maximum Ca current can be tremendous. A non-linearity between Vmax and gCa has been described. Furthermore, a twice peaking of the calcium current during CaAP cannot be excluded. The existence of outward currents in nearly the same size as the Ca current even during the upstroke of CaAP seriously question the use of CaAP as a measure of calcium current properties.