We have examined the stability and sources of variation within the nociceptive model of rat hind paw withdrawal from an under-glass radiant stimulus (Hargreaves et al., 1988) using a system where stimulus intensity and floor temperature can be controlled and reproducibly changed. The current study demonstrates that: (i) increased stimulus intensity with a fixed surface temperature is associated with a monotonic decrease in mean response latency and its variance; (ii) for a fixed stimulus intensity, the mean paw withdrawal latency and variance increased as the glass floor temperature is lowered from 30 degrees C to room temperature (25 degrees C). Using subcutaneously-implanted thermocouples and a 30 degrees C glass surface, the subcutaneous paw temperature observed at an interval corresponding to the time at which the animal displayed a paw withdrawal did not differ across multiple heating rates (41-42.5 degrees C). This finding is in agreement with human studies of pain thresholds and C-fiber activity. These studies emphasize the importance of maintaining a fixed surface temperature to reduce experimental variability and the utility of this apparatus across multiple stimulus intensities to define agonist efficacy.