A series of bioassay experiments concerning the effects of heated water on white perch have shown that, for water temperatures ranging between 82°F (300.9°K) and 97°F (309.3°K), the lethal effects on a white perch being held at any temperature within that range will occur within the first 36 hrs of exposure. This is shorter than the time period prescribed in general by the standard bioassay procedure. Three parameters are given which more completely describe the pattern of mortality. Investigation has shown that: (1) A relationship exists between the temperature at which mortalities of the white perch are incipient and the temperature at which the body of the fish is functioning and (2) the lethal effects on white perch caused by extreme water temperatures are independent of the rate of temperature increase.