Kinetics of oxidation of L-citrulline (Cit) by permanganate ion in both acidic and basic media has been investigated spectrophtometrically at constant ionic strengths and at 25°C. In both media the reactions exhibited first order dependence in [permanganate] and less than unit order dependences in L-citrulline concentration. A fractional-second order dependence with respect to [H+ ] and a fractional-first order dependence with respect to [OH- ] were revealed in acidic and basic media, respectively. Increasing ionic strength in basic medium increased the oxidation rate of L-citrulline, whereas it had a negligible effect on the oxidation rate in acidic medium. The rate-determining step in both media is suggested to involve a oneelectron change, but the stoichiometry (L-citrulline: permanganate) was different, being 5:2 in acidic medium and 1:2 in basic medium. The proposed oxidation mechanisms involve formation of 1:1 intermediate complexes between kinetically active species of both L-citrulline and permanganate ion in pre-equilibrium steps. The final oxidation products of L-citrulline were identified in both acidic and basic media as the corresponding aldehyde (4-(carbamoylamino) butyraldehyde), ammonia and carbon dioxide. The appropriate rate laws are deduced.