1. Phase response curves for 15' bright light pulses of four species of nocturnal rodents are described. All show delay phase shifts early in the subjective night, advance shifts in the late subjective night, and relative insensitivity during the subjective day. 2. The broad scatter in measured phase-shifts is largely due to error of measurement: the response of the pacemakers to light stimuli is more accurate than we observe. 3. Indications are found that the response to a resetting stimulus at a given phase of the rhythm is correlated with the individual ¯τ (freerunning period). Fast pacemakers (short ¯τ) tend to be more delayed or less advanced by the light than slow pacemakers (long ¯τ). 4. Within individual mice (Mus musculus) the circadian pacemaker adjusts its resetting response to variations in its frequency: when τ is long (induced as after-effect of prior light treatment) light pulses at a defined phase of the oscillation (ct 15) produce smaller delay phase shifts than when τ is short. 5. Among species there are conspicuous differences in the shape of the phase response curve: where ¯τ is long advance phase shifts are large and delay phase shifts small (Mesocricetus auratus); where ¯τ is short, advance shifts are small, and delay shifts are large (Mus musculus; Peromyscus maniculatus). 6. The functional meaning of the interrelationships of τ and PRC is briefly discussed.