Groups of 30 male Sprague-Dawley rats were given 4-nitrosomorpholine-3,3,5,5-d₄ in their drinking water at concentrations of 0.35 and 0.07×10⁻³ M for 30 weeks. Two similar groups of rats were simultaneously given unlabeled 4-nitrosomorpholine (NM) at the same molar concentrations; all animals were observed throughout their lives. Those receiving the α-deuteriumlabeled compound had significantly fewer liver tumors than did the corresponding animals receiving the unlabeled compound. The difference in potency appeared to be at least fivefold, a magnitude consistent with a primary kinetic isotope effect on the carcinogenic action of NM. Thus breakage of a bond linking a hydrogen (deuterium) atom with a carbon adjacent to the nitrosamino function may be involved in a rate-limiting step of carcinogenesis by NM.