The fishes used in these expts. were Fundulus parvipinrris and Gillichthys mirabilis. The anesthetics used were urethane, chlore-tone and ether, the lethal agents NaCN, KCN, asphyxiation and excessive temp. Fishes kept at widely different temps., when tested at these temps., were found to differ markedly in the rate of their visible respiratory movements, this being in general higher at higher temps. Susceptibility to urethane was also higher at higher temps. When fishes which had become acclimatized to high and low temps. were transferred to a common temp., those from the warmer water displayed a lower respiratory rate and lower susceptibility to anesthetics and lethal agents than those from colder water. Thus the original differences were reversed. Acclimatization to a high temp. appears to consist in part, at least, in a change whereby an initial great increase in metabolic rate is followed by a regulative process, with continued sojourn in warm water; and conversely, acclimatization to a low temp. involves a similar process in the opposite direction. Differences in susceptibility to urethane seem to have been brought about by 24 hrs.'' exposure to the differing temps.; differences in susceptibility to KCN or oxygen-free water required much longer exposure. Induced differences in metabolic rate persisted, in one case, for 14 days after removal of the environmental cause.