In a series of experiments, male and female Sprague Dawley rats, kept in light (L) from 06 time in hours to 18 time in hours alternating with darkness (LD 12:12) inhaled different concentrations of carbon monoxide (50-1,700 ppm) at each of two test times, 12h apart. A decrease in flow of CO2 (V co2) resulting from CO inhalation was greater in the active dark (D) than resting light (L) span. Experimental hypoxic mortality of male and female mice also shows circadian variations, being greater in the D than in the L span. Moreover, a difference of mortality was observed between hypoxic exposures performed at 12 time in hours (in LD or DL) and hypoxic exposures performed at 00 time in hours (in LD or DL). Such results await tests of any extent to which they model responses of human beings to air pollution. In human beings any external environmental circadian, circaseptan, and circannual variations in air pollution as such may serve to a variable extent as socioeconomic synchronizers of innate rhythms with a corresponding frequency, rather than as solely generators of time patterns in any physiopathologic response to air pollution.