To investigate the involvement of the medullary raphé in thermoregulatory vasomotor control, we chemically manipulated raphé neuronal activity while monitoring the tail vasomotor response to preoptic warming. For comparison, neuronal activity in the rostral ventrolateral medulla (RVLM) was manipulated in similar experiments. Injections of D,L-homocysteic acid (DLH; 0.5 mM, 0.3 microl) into a restricted region of the ventral medullary raphé suppressed the tail vasodilatation normally elicited by warming the preoptic area to 42 degrees C. DLH injection into the RVLM also suppressed the vasodilatation elicited by preoptic warming. Injection of bicuculline (0.5 mM, 0.3 microl) into the same raphé region suppressed the vasodilatation elicited by preoptic warming. Bicuculline injection into the RVLM did not suppress tail vasodilatation. These results suggest that neurones in both the medullary raphé and the RVLM are vasoconstrictor to the tail, but only those in the raphé receive inhibitory input from the preoptic area. That input might be direct and/or indirect (e.g. via the periaqueductal grey matter).