Voltage-gated sodium channel currents were recorded in cell-attached and inside-out membrane patches from rat ventricular myocytes at temperatures ranging from 4 degrees C to 36 degrees C. The effects of changes in temperature on channel conductance were accurately determined using a digital signal processing technique based on hidden Markov models. We show that the cardiac sodium channel has multiple conductance levels, with the most frequently observed sublevel being either 11 pS or 22 pS at room temperature in 280 mM Na+, depending on the dissociation procedures adopted. The single channel current-voltage relationship is ohmic at all of the temperatures studied. The conductance increases steeply with temperature, with Q10 ranging from 1.4 to 1.5. The proportional change in channel conductance with increasing temperature was greater than the corresponding change in bulk conductivity of electrolyte solutions, suggesting that an ion traversing the channel needs to surmount a small additional energy barrier. An activation energy deduced from a plot of the logarithm of single channel conductance against the inverse of temperature is about 28 kJ mole-1. We provide one possible interpretation of the observed conductance-temperature relationship in terms of the details of the microscopic interactions operating between the protein wall, ions and water molecules.