The mutationno action potentialtemperature itive(napts) induces arrhythmia in the heartbeat ofDrosophila melanogasterlarvae at temperatures above 20°C; heartbeat becomes normally rhythmic again after a shift back to 20°C. For this phenotype,naptsis almost completely recessive to the wild type.naptsalso reduces the temperature-sensitivity of heart rate over a wide range of temperatures, for this phenotype,napts, is dominant over the wild type,naptscauses reversible paralysis in adults by epistatic effects on the expression ofparalytic, a gene encoding a voltage-dependent sodium channel. However, theparamutation, which induces paralysis in adults at 29°C, has no effect on larval heartbeat at temperatures between 20°and 37.5°C. Theperiodgene,contraearlier reports, has no effect on heartbeat.