Heart rate in pre-pupae of Drosophila melanogaster is shown to vary over a wide range from 2·5 to
3·7 beats per second. Quantitative genetic analysis of a sample of 11 highly inbred lines indicates
that approaching one-quarter of the total variance in natural populations can be attributed to
genetic differences between flies. A hypomorphic allele of the potassium channel gene ether-a-gogo,
which is homologous to a human long-QT syndrome susceptibility gene (HERG), has a heart rate
at the low end of the wild-type range, but this effect can be suppressed in certain wild-type genetic
backgrounds. This study provides a baseline for investigation of pharmacological and other
physiological influences on heart rate in the model organism, and implies that quantitative genetic
dissection will provide insight into the molecular basis for variation in normal and arrhythmic
heart function.