The initial pharmacokinetic observations on ethylene oxide (EO) were made by Ehrenberg et al. in mice. There are two reasons for suspecting that the mouse may not be representative of other species in its handling of EO. Metabolism of EO leads to the formation of S-carboxymethylcysteine. Using a closed inhalation chamber system, studies have been carried out in male Sprague-Dawley rats of the pharmacokinetics of ethylene, as a precursor of endogenously formed EO, and of EO as an exhaled reactive metabolite of ethylene. The conclusion was that "although ethylene definitely represents a genotoxic hazard", its low rate of biotransformation to EO, coupled with the distinct saturation pattern of this transformation, precludes the possibility of observing statistically significant tumorigenicity on long-term exposure of rats to ethylene. In the absence of human data on the metabolism and pharmacokinetics of EO, one cannot decide whether the apparently wide divergence between rat, mouse, and dog has important practical implications.