Rhythm of activity of inbred mice was recorded automatically by a set of actographs. This rhythm was characterized by six indices in the temporal and frequency domains. Two methods of genetic analysis were applied to these indices using parental strains BALB/c and C57BL/6, the reciprocal F1's, and the seven recombinant inbred strains (RI). Findings on the F1's show no maternal effect but indicate dominance and heterosis. The RI method successfully rejects the hypothesis of a monogenetic correlate for all measures. In line with F1 data, it demonstrates the presence of a polygenetic correlate: at least one other locus is involved in each of the six outcome parameters.