An attempt has been made to study the quantitative relationship between the temperature of the anterior hypothalamic/preoptic (AH/PO) region and heat balance in the conscious ox (Bos taurus) at a thermoneutral air temperature of 20 degrees C. Heat balance was measured by combined gradient-layer and open-circuit calorimetry and AH/PO temperature was controlled using a water-perfused thermode. Continuous warming or cooling of the AH/PO region caused a transient heat imbalance which shifted core temperature to a new stable level dependent upon the level of stimulation. The heat balance responses to continuous AH/PO warming included peripheral vasodilation and increased respiratory and cutaneous evaporative heat loss; those to AH/PO cooling consisted mainly of decreased heat loss. Heat production was largely unaffected by both AH/PO warming and cooling. The heat balance responses developed only slowly, reaching a peak up to 50 min after the start of stimulation and taking several hours to complete. The change to stability in core temperature during continuous AH/PO stimulation and the additional heat loss prior to that stability were the most sensitive indicators of response of all the variables measured. A significant response was observed in these parameters even at the smallest stimulus level (less than 0.33 degrees C) and correlation of response with stimulus level was precise with a zero threshold for response. This suggests a continuity of operation of the AH/PO mechanism about zero. Several responses showed a curved relationship between slope of response and stimulus level, such that the increment in response per unit increase in stimulus level became greater as stimulus level increased. The results suggest a two-tier form of thermoregulation in the ox with AH/PO sensitivity to warming greater than its sensitivity to cooling. It appears also that AH/PO temperature is at least as dominant in the thermoregulatory mechanisms in the ox as it is in smaller animals.