1. The micro-iontophoretic technique has been used to study the responses of single neurones in the bulbar reticular formation to 5-hydroxytryptamine and to noradrenaline, ACh or glutamate, and to compare these with the responses to electrical stimulation in or near the bulbar raphe nuclei. 2. In the bulbar reticular formation, most neurones were excited by 5-hydroxytryptamine; forty-three of fifty-one neurones excited by 5-hydroxytryptamine were also excited by stimulation in nucleus raphe magnus, nucleus raphe pallidus or nucleus raphe obscurus. Most of these stimulation-induced excitations were of long latency: LSD reduced six of seven of these excitations tested, while 5-hydroxytryptamine excitations were blocked on all seven. 3. In contrast, stimulation of areas adjacent to the raphe nuclei excited only fifteen of forty-six neurones excited by 5-hydroxytryptamine. Most of these stimulation effects were of short latency and none of the three tested were reduced by LSD, although 5-hydroxytryptamine excitations were blocked. 4. The relationship of the long-latency excitatory effects of stimulation with the position of the stimulating electrode in the raphe nuclei indicates that these effects are probably mediated via the raphe neurones and this is supported by the correlation of the effects of raphe stimulation with the effects of 5-hydroxytryptamine applied iontophoretically and by the ability of LSD to block both effects. 5. The results provide a physiological basis for the excitatory effects of iontophoretically applied 5-hydroxytryptamine in the bulbar reticular formation.