The construction of a horizontal temperature gradient is described in detail. The apparatus is built from readily available and inexpensive materials. The chamber is built from PVC tubing, fitted with an aluminum floor, and placed in a sound-attenuated box. A temperature gradient is formed by placing solid CO2 at one end and a hot plate at the opposite end of an aluminum floor. The apparatus described is reliable, dependable, and has proven to be very suitable for use with small rodents.