The circadian wheel-running rhythm of golden hamsters was monitored du
ring chronic oral treatment with four mood-stabilizing drugs in doses
relevant for treatment of manic-depressive disorder. Carbamazepine and
verapamil shortened the duration of locomotor activity and improved t
he stability of the pattern of running activity. At comparable doses,
valproate had no clear effect on any rhythm variable tested. None of t
hese drugs consistently altered the phase of light-synchronized runnin
g rhythms or the period of the rhythm in constant darkness. The result
s are compared to data showing that lithium, which delays entrained ph
ase and lengthens circadian period in hamsters, also shortens activity
duration and increases the stability of wheel-running rhythms. Stabil
ization of circadian rhythms may be a key action of clinically effecti
ve mood-stabilizing drugs.