We measured heart rate and rhythmicity (regularity) of heartbeat in Drosophila melanogaster at five different temperatures (20, 25, 30, 35, and 37 degrees C) for a Florida population and estimated the narrow-sense heritability of both traits. Heritability of heart rate ranged from 0.17 to 0.24, but was statistically significant only at 20 degrees (h(2)=0.24) and at 30 degrees (h(2)=0.23). The heritability of heartbeat rhythmicity ranged from -0.034 to 0.11, and was not significant at any temperature. Heart rate increased linearly with increasing temperature; the temperature-dependence of heart rate was itself heritable (h(2)=0.29). Heart rhythmicity varied curvilinearly and was well-represented by a parabolic function, peaking at about 27 degrees which suggests a temperature optimum. The regularity of the heartbeat did not covary with heart rate except at 20 degrees . Neither heart rate nor regularity covaried with the change in heart rate with temperature. For this population of D. melanogaster, we conclude that there is substantial genetic variation for the mechanism whereby the cardiac pacemaker reacts to changes in temperature, but not for the cardiac pacemaker's rhythmicity. The small values of h(2) for temperature-specific heart rate and heartbeat rhythmicity suggest that these traits have been subjected to natural selection.