Ts65Dn mice are used extensively as a model for Down syndrome. Recent studies have reported conflicting evidence as to whether these mice express circadian rhythms. The authors therefore recorded locomotor activity patterns from these animals while they were housed under a standard light-dark cycle, constant darkness (DD), and constant light (LL). Contrary to expectations, Ts65Dn mice had more robust circadian rhythms with slightly shorter periods compared with their wild-type littermates. They also exhibited increased rhythm period and marked activity suppression when moved from DD to LL (i.e., Aschoff's rule). Administration of the GABA(A) antagonist pentylenetetrazole did not influence any of these circadian parameters. Thus, locomotor activity is under strict circadian control in Ts65Dn mice, suggesting that their cognitive deficits and sleep disturbances are not due to dysfunctional circadian timing as proposed previously.