1. 1.|Changes in fusimotor activity were observed during local cooling or warming of the midbrain reticular formation (MRF) of urethan-anaesthetized rats, whose bilateral preoptic and anterior hypothalamus were electrolytically lesioned. 2. 2.|At the normal MRF temperature, 23 (36%) of the 64 fusimotor fibres isolated were found to exhibit periodic fluctuations in spontaneous discharge whereas the nest sustained spontaneous discharge. 3. 3.|Of 38 fusimotor fibres examined, 27(71%) were affected by changes in local temperature of the MRF. In most (93%) of these fibres, the discharge frequency was increased by cooling of the MRF below normal body temperature (37.5°C) and decreased by heating of the MRF above this temperature. In the temperature reactive fusimotor fibres, temperature dependencies of discharge activity were obtained; −1.5 ± 0.1 imp./ s °C (< 37.5°C), 2.8 ± 0.5 imp./ s °C (> 37.5°C). 4. 4.|Results indicate that the fusimotor activity is enhanced in the hypothermic range and suppressed in the hyperthermic range, and suggest that the descending signals derived from the thermo-sensitive structures in the MRF control shivering threshold at the spinal level.