In order to determine the factors affecting the maturation of the functional hypothalamo-pituitary-thyroid axis, Ambystoma gracile (Baird) larvae of different ages and acclimated to different temperature regimes were exposed to a concentration of thyroxine (T4) which activates the pituitary-thyroid axis. A. gracile from a montane and low-altitude population were utilized. These studies, plus observations concerning spontaneous metamorphosis, indicated that populations of Ambystoma gracile are highly polymorphic as suggested by Sprules (1974 b). A. gracile populations consist of 'obligate transformers', 'obligate neotenes', and 'facultative transformers'--which will metamorphose depending on environmental conditions. The colder high-altitude conditions tend to select against facultative individuals, whereas the less certain low-altitude conditions permit a much higher proportion of animals that exhibit a facultative response with respect to metamorphosis. Interrelationships between environment and physiological parameters that determine morphology in salamanders are discussed with regard to these findings.