The morphology and the immuno-distribution ofthe inducible isoform of nitric oxide synthase (iNOS) havebeen examined in regenerating tails from differently agedXenopus laevis larvae. By comparing stage-50 and stage-55/56 tadpoles, various morphological aspects and immunoreactivityto anti-iNOS antibody in terms of the numberand duration of positive cells have been demonstrated in theregenerating buds. Unlike in stage-50 larvae, the extent ofresponses to tail amputation in older larvae is moredependent on the individual tadpole and a high percentage(70%-80%) of malformed tails has been seen. The findingsindicate that the decline in the efficiency of Xenopus tailregeneration is driven by differences in the inflammatoryresponses and in the involvement of nitric oxide. Thismolecule is induced and required for normal tail regeneration,whereas in excess, it is probably associated withprogressive loss in the regeneration capability.