Supersonic or hypersonic aircraft flight can produce hyperthermia in crew members. Dark adaptation is a physiologic response possibily influenced by such stress. Biochemical regeneration rate of rhodopsin might be altered if retinal blood or tissue temperatures are raised by this hyperthermia. Peripheral dark adaptation curves were obtained on five subjects at comfort (21.0 C) and 65.6 C temperatures alternately, using a modified Hecht-Schlaer adaptometer. In 24 heat experiments, subjects were exposed to 65.6 C ambient air temperature for 55 min after which the heat was shut off. Chamber temperature declined but the subjects remained in a relatively constant hyperthermic condition during the subsequent 35-min dark adaptation test. The criterion used here for hyperthermia is the elevation and maintenance of the rectal temperature at least 0.56 C above corresponding control temperature. Curves obtained under hyperthermic conditions (with the exception of the initial and terminal values) differ, with statistical significance, from controls, indicating a facilitation of dark adaptation under the imposed thermal stress conditions.