This experiment examined the effect of destruction of the ascending 5-hydroxytryptaminergic (5HTergic) pathways on performance in a free-operant timing schedule: the "time-left" procedure. Rats received either injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were trained in a discrete trials schedule in which reinforcers were provided for responding on either of two levers, A and B. At a random time point, t s after the start of each trial, a response on A resulted in the delivery of one food pellet after dA s, whereas a response on B resulted in the delivery of two pellets after 60-t s. The value of dA was varied between 1 and 8 s in different phases of the experiment. Both groups showed decreasing response rates on lever A and increasing response rates on lever B as a function of time within the trial. An index of timing (T75: the time within the trial at which relative response rate on B attained a value of 75%) was systematically related to the value of dA, but did not differ significantly between lesioned and control rats. However, the lesioned group showed significantly higher rates of switching between response alternatives than the sham-lesioned group at all values of dA. The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not significantly altered. The results provide further evidence that the ascending 5HTergic pathways may contribute to the inhibitory regulation of switching between behavioural states.