The free running period (tau) of male Syrian hamsters, when measured as the circadian rhythm of emergence from simulated burrows, was compared to tau when locomotor activity was measured via running wheels. In constant darkness, the inter-individual variability of the tau s was less among burrow housed hamsters than for running wheel housed hamsters. Hence, access to activity wheels eventually led to more disparate activity onset times among the running wheel animals, as compared to the relative population synchrony observed when animals were housed in simulated burrows. When tau s were contrasted between hamsters housed in standard cages with or without running wheels, it appeared that tau values of hamsters housed in simulated burrows were more similar to those of hamsters housed in standard cages without wheels. The data suggests that the tau of the hamster circadian system may be subject to subtle environmental influences.