1. Twenty-six shorn Scottish Blackface sheep received nine daily cold shocks (down to −10°C) each lasting 1 hr. This treatment reduced the metabolic response to cold (lower heart rates) and caused passive body cooling—defined as habituation. Eight selected sheep showing maximum habituation and 16 untreated controls then received blocking treatment (re-shorn and cooled for 2 hr at 18°C or 8°C) before being heat-stressed (at 42°C) to elicit panting.
2. Thermal panting, delayed as expected by blocking treatment, was further delayed by previous cold habituation. Habituated sheep blocked at 18°C panted 30 min later than blocked controls. In habituated sheep blocked at 8°C, panting was completely inhibited even after 90 min heat stress; their final mean respiration rate was 19/min v. 220/min in blocked controls and 250/min in unblocked controls. Rectal temperatures of habituated sheep were lower than controls during blocking treatment.
3. Habituation, by restraining the normal metabolic response to cold, increased the heat debt during blocking treatment and lessened the heat load during subsequent heat stress. This apparently reduced the drive for panting. Habituation may also have favoured a direct inhibitory effect of cold on the respiratory centres.