1. The influences of ambient temperature (T(a)) on the thermoregulatory effector activities and the body temperature (T(b)) of intraventricular injections into the sheep, goat and rabbit of 5-hydroxytryptamine (5-HT), noradrenaline (NA), acetylcholine (ACh), carbachol and eserine, have been interpreted in terms of a simple neuronal model of the pathways between thermosensors and thermoregulatory effectors.2. In all three species 5-HT in minimal doses caused a rise in respiratory frequency (RF) and a fall in T(b) at high T(a), and a reduction in EMG activity and a fall in T(b) at low T(a). These effects could be interpreted as those of an excitatory transmitter acting on the warm receptor-heat loss pathway.3. In all three species NA caused a reduction in RF and a rise in T(b) at high T(a), and a reduction in EMG activity and a fall in T(b) at low T(a). These effects are interpreted as those of an inhibitory transmitter acting both on the warm sensor-heat loss pathways and on the cold sensor-heat production pathway.4. The effects of ACh and the cholinomimetic substances carbachol and eserine are complex and more difficult to interpret. In small doses the effects on the sheep and goat are those of an excitatory transmitter on the cold sensor-heat production pathway. There was an increase in EMG activity and a rise in T(b) at low T(a), and a reduction in RF and a rise in T(b) at high T(a). At higher dose levels in the goat and at all dose levels in the rabbit these substances had the reverse effects which are attributed to a synaptic block due to the excess of the excitatory substance.5. The effects of ambient temperature and injected substances upon ear temperature are consistent with the predictions of the model if it is assumed (a) that at high and low ambient temperatures direct thermal effects on ear vessels dominate those of the sympathetic innervation, and (b) that the warm sensor influence is to lower peripheral vasomotor tone, and the cold sensor influence is to increase it.6. The conclusion reached is that when consideration is given to species differences in the thermoneutral ambient temperature and to the possibility that excitatory substances have reversed effects at high dose levels, the effects of 5-HT, NA and ACh in the control of body temperature are very similar in the sheep, goat and rabbit: 5-HT is excitatory on the heat loss pathway, ACh is excitatory on the heat production pathway and NA has an inhibitory influence on both pathways.