Seven male and seven female rat pups, all from separate litters of 10 pups each, had biotelemetry thermistors implanted into their peritoneal cavities when they were 10-15 days old. Body temperatures (Tb) were recorded by an Apple computer every 10 minutes until the rats were 60-75 days old so that the development of the circadian temperature rhythm (CTR) could be studied. The data were compared with eight parameters taken from adult CTRs. All rats were maintained on a 12 hr light/12 hr dark photoperiod. The results indicate that the CTR begins to develop after thermoregulatory mechanisms mature to produce an adult like daily range of TbS around day 24. From days 25 to 45, there was a steady increase in the percentage of TbS greater than the daily mean occurring in the dark. This process organized Tb into the characteristic sinusoidal waveform of the adult. Proper phasing of the trough and peak relative to the light/dark cycle took place between days 30-36. By day 42, 90% of TbS higher than the daily mean were occurring in the dark, while those lower than the mean were in the light. This is the typical adult pattern. The waveform's appearance continued to develop until day 50.