Mice are generally housed in groups in cages lined with an absorbent bedding material at ambient temperature (Ta) of 20 to 24 degrees C, which is comfortable for humans, but cool for mice. Little is known about the effects of bedding on thermoregulation of group-housed mice. To determine whether bedding material affects thermoregulatory stability, core temperature (Tc) and motor activity (MA) were monitored by use of radiotelemetry in female CD-1 mice housed in groups of four in a standard plastic cage at Ta of 23.5 degrees C. Ten groups were tested using three types of bedding material: a deep layer of heat-treated wood shavings (DWS) that allowed mice to burrow, a thin layer of wood shavings (TWS) just covering the bottom of the cage floor, or a layer of beta chips (BC). Mice could not burrow in the TWS or BC. The Tc and MA were affected by bedding type and time of day. Mice housed with DWS maintained a significantly higher Tc (deltaTc = 1.0 degrees C) during the day, compared with that in mice housed with TWS and BC. During the night, Tc and MA were high in all groups and there was no effect of bedding type on Tc or MA. Effect of bedding on metabolic rate (MR) was estimated by measuring oxygen consumption for six hours in groups of four mice at Ta of 23.5 degrees C. The Tc was significantly reduced in mice housed on the TWS and BC, but MR was unaffected by bedding type. There was a trend for higher MR in mice on BC. Compared with use of other bedding materials, housing mice on DWS and comparable materials provides an environment to burrow, thus reducing heat loss. The effects of bedding material on temperature regulation may affect rodent health and well being. Moreover, bedding will affect variability in toxicologic and pharmacologic studies whenever an endpoint is dependent on body temperature.