The toxicity of cadmium to rainbow trout fry resulting from brief or continuous exposure is compared. Results of tests involving brief cadmium exposure followed by restoration to clean water indicate that postexposure mortalities may result from contact for as little as 32 min to 1.0 mg Cd liter-1 or 10 min to 10 mg Cd liter-1. These results highlight the inadequacy of standard toxicity tests in predicting the consequences of an episodic pollution incident such as that resulting perhaps from an accidental spillage. For example, the median lethal time (LT50) following continuous exposure to 1.0 mg Cd liter-1 is 1900 min whereas exposure for only 32 min will ultimately result in 50% mortality. The term median postexposure lethal time (peLT50) is proposed as a means of assessing and comparing the results of brief exposure to a pollutant and it is suggested that brief or episodic exposure tests should be incorporated into the hazard evaluation process.