The tau mutation in the golden (Syrian) hamster is a single gene mutation that drastically affects the speed of the circadian clock, in such a way that homozygous mutants have an endogenous circadian period of 20 h (compared with 24 h for wild-type hamsters). While studying the circadian system of tau-mutant hamsters during the past 25 years, several authors have noted an apparent relationship between circadian period and body size in these animals. This study, based on 181 hamsters from 24 litters, confirmed previous observations that a shorter circadian period is associated with smaller body size, documented a sex difference in this association, and evaluated several mechanisms that might explain the phenomenon (such as different organ sizes, body composition, and metabolic rate). The obtained evidence suggests that the reduced body size of short-period hamsters is likely a pleiotropic effect of the tau allele (an allele of the casein kinase 1 epsilon gene) rather than a consequence of the shortened circadian period.