The growth and histological features of the tail vertebrae of young mice (31 day old) were observed for 48 h after transference from a cold (8 degrees) to a hot (33 degrees) environment. Some mice received a massive (5000 rad) dose of X-ray to the tail. Less than 2 h after housing the animals in the heat there was a period of very rapid growth lasting 12 h after which time growth slowed but was still much greater than the animals in the cold. Animals whose tails were irradiated (and also put in the heat) showed the same rapid growth for 12 h but after this growth ceased. Evidence is provided which shows that the initial rapid growth in the heat is due to an abnormal increase in the linear dimensions of the hypertrophic cells of the growth plate. This is most marked in the hot-irradiated animals
