The importance of light and stress as factors in growth has not previously been clearly delineated. As the result of using experimental environments on growth rate of incisors in 24 young rats with body weight ranging 58 Gm to 62 Gm., the author obtained following conclusions. 1) The eruption rate of normal rats incisors was 0.421mm in upper incisors and 0.592mm in lower incisors per a day respectively. 2) In light environment, growth rate of incisors in rats and body weights were found to increase more rapidly during the 8-week experimental period in comparison with any environments. 3) In stress environment, growth rate of incisors in rats and body weight were found to decrease in comparison with amy environments during the 8-week experimental period.