Adult per⁰¹ flies expressing per^{S826A.S828A.T:Ivir\HA1,T:Zzzz\His6} (rescue flies) display robust circadian locomotor activity rhythm in constant dark (preceded by an 12:12 light:dark entrainment period) with a period of ~23 hr - about 1 hr shorter than wild-type, the upswing in the morning activity peak occurs earlier compared to controls but the timing of the evening activity bout is not significantly affected. When entrained on a short photoperiod regime ( 9:15 light:dark cycle), the timing of both evening and morning peaks is early, substantially so (by ~1.4 hr) for the morning one. The per⁰¹;per^{S826A.S828A.T:Ivir\HA1,T:Zzzz\His6} flies also show defects in the temperature compensation of period length: their free-running locomotor activity periods shorten with the rising temperature. However, neither the cycling pattern of per mRNA expression nor the timing of the per protein nuclear entry is perturbed in these mutants and they also do not display any defects in photic entrainment abilities.