The internal gestation and subsequent lactation of mammalian reproduction represent a considerable physiological challenge. The extent of disruption in the daily rhythm of four parameters, core body temperature, nest attendance, activity, and wheel running, was monitored in Djungarian hamster (Phodopus campbelli) and Siberian hamster (P. sungorus) females implanted intraperitoneally with biotelemetric thermistors. The amplitude of each rhythm decreased during late gestation, culminating in a substantial disruption at parturition, and did not begin a recovery until the latter third of lactation. In each species, the change in the core body temperature rhythm was primarily the result of an elevation in light phase body temperature to approximate the normally occurring dark phase temperature, although the disruption was more extensive in P. sungorus than in P. campbelli. As this maternal hyperthermia is associated with the provision of essential heat to the altricial liter, these species differences in the vulnerability to hyperthermia may constrain the reproductive success of these extreme cold adapted small mammals.