The effects of changing temperature on the electrophysiology of isolated cardiac myocytes of the guinea pig and Richardson's ground squirrel were studied by patch-clamp techniques. In cells from both species, the resting membrane potential declined on cooling from 36 to 12 degrees C by approximately 6 mV. The duration of the plateau of the action potential in guinea pig cells increased monotonically on cooling. In contrast, the action potential of ground squirrel cells showed a biphasic response, increasing in duration from 36 to 24 degrees C and then decreasing on cooling from 24 to 12 degrees C. From voltage-clamp studies, the properties of L-type calcium currents (ICa) on cooling were compared in the two species and were found to be similar: In both cases, ICa decreased in amplitude from approximately 2 nA peak current at 36 degrees C to less than 400 pA at 12 degrees C. The Q10 of both the maximum amplitude and time to peak for ICa in both species was approximately 1.8. The time for half inactivation had a greater Q10 of 2.5-3. It is concluded that, surprisingly, factors affecting the resting membrane potential and properties of L-type calcium channels are not major contributors to cardiac dysfunction on cooling. Rather, it is sarcoplasmic reticulum calcium release and reuptake that are likely to be the most important cold-sensitive processes.