The suprachiasmatic nucleus (SCN) controls the timing of the preovulatory LH surge in laboratory rodents. Barbiturate administration during a critical period on proestrus delays the surge and prolongs the estrous cycle one day. Because a non-photic timing signal (zeitgeber) during the critical period that phase advances activity rhythms can also induce the latter effect, we hypothesized that barbiturates delay the LH surge by phase-advancing its circadian timing signal beyond the critical period. In Experiment 1, locomotor rhythms and estrous cycles were monitored in hamsters for 2-3 weeks pre- and post-injection of vehicle or phenobarbital and post transfer to darkness at zeitgeber time (ZT) 6 on proestrus. Phenobarbital delayed estrous cycles in 5 of 7 hamsters, which exhibited phase shifts that averaged two-fold greater than those exhibited by vehicle controls or phenobarbital-injected hamsters with normal cycles. Experiment 2 used a similar protocol, but injections were at ZT 5, and blood samples for LH determination were collected from 1200-1800 h on proestrus and the next day via jugular cannulae inserted the day before proestrus. Phenobarbital delayed the LH surge one day in all 6 hamsters, but it occurred at an earlier circadian time, supporting the above hypothesis. Experiment 3 investigated whether phenobarbital, like other non-photic zeitgebers, suppresses SCN Period1 and Period2 transcription. Two hours post-injection, phenobarbital decreased SCN expression of only Period1 mRNA, as determined by in situ hybridization. These results suggest that phenobarbital advances the SCN pacemaker governing activity rhythms and hormone release in part by decreasing its Period1 gene expression.