1. The effects of noradrenaline (NA), adrenaline, dopamine (DA) 5-hydroxytryptamine (5-HT) and a number of related drugs were tested on the extracellularly recorded responses of neurones in the feline thalamus. Substances were applied iontophoretically and tested on synaptically, antidromically and chemically evoked neuronal activity.2. NA, adrenaline, isoprenaline and 5-HT had a variety of effects, depressing some cells, exciting others and not affecting the responses of a third group. DA depressed most of the cells tested; excitation was not observed with this compound.3. The magnitude of depressant actions and their duration varied considerably. The more sensitive cells responded to extremely small amounts of catecholamine or 5-HT and recovery often took several minutes. Recovery after DA was always rapid. Neurones in the dorsal thalamus were generally more susceptible to depression than those in the ventro-basal complex.4. Excitatory responses were most marked in the ventro-basal complex of the thalamus. Desensitization occurred if NA or adrenaline was applied repeatedly and this tachyphylaxis lasted for several minutes. After desensitization to the excitatory effects, some of these cells were depressed by catecholamines. These findings suggest the presence of at least two types of membrane receptor.5. beta-adrenergic antagonists (alderlin, D-INPEA and MJ 1999) and alpha-antagonists (phentolamine, dibenzyline and chlorpromazine) had pronounced depressant actions on some thalamic neurones. With the exceptions of D-INPEA and MJ 1999 they also excited cells that were excited by the catecholamines. Alderlin and phentolamine had both excitatory and inhibitory effects on some cells.6. The monoamine oxidase inhibitor, iproniazid, depressed neurones which were sensitive to NA depression. It did not appear to potentiate the effects of NA on most of the cells tested.7. Reticular formation stimulation depressed some neurones in the thalamus and excited others. The depressant effects of NA and reticular formation stimulation were reduced or abolished by an intravenous injection of picrotoxin (1 mg/kg).8. It is suggested that NA and 5-HT may be inhibitory transmitters in the thalamus, released at the terminals of ascending pathways from the brain stem that have been defined by fluorescence microscopy. The excitatory actions of these compounds may also be related to a synaptic role.