The idea of ameliorating jetlag with drugs has received considerable attention. Melatonin has been found to reduce feelings of jetlag in people after transatlantic flights. In hamsters, injections of triazolam, a benzodiazepine, increase the rate of adjustment of activity rhythms to an 8 h advance of the light-dark (LD) cycle. But melatonin can make people drowsy and triazolam often induces hamsters to run in their wheels. Therefore, it is not clear whether these chemicals exert their chronotypic effects by acting directly on circadian pacemakers or because they first alter behavioural states. Non-photic behavioural events (for instance, social interactions) are capable of entraining rhythms and causing phase shifts. Thus, it is possible that behavioural events alone could alter the rate of adjustment to new LD cycles. To investigate this possibility, we studied the rate of re-entrainment of hamsters in a testing paradigm similar to that used with triazolam. We found that the rate of adjustment could be more than doubled simply by making the animals active on a single occasion in the middle of their normal rest period, immediately after the shift in the LD cycle.