Nitric oxide (NO) is synthesized from arginine by nitric oxide synthase (NOS), and citrulline which is generated can be recycled to arginine by argininosuccinate synthetase (AS) and argininosuccinate lyase (AL). Rats were injected with bacterial lipopolysaccharide (LPS), and expression of the inducible isoform of NOS (iNOS), AS, and AL was analyzed. In RNA blot analysis, iNOS mRNA was undetectable before the LPS treatment but was induced by LPS in the lung, heart, liver, and spleen, and less strongly in the skeletal muscle and testis. AS mRNA was induced in the lung and spleen, and AL mRNA was weakly induced in these tissues. AS and AL mRNAs were abundant in the control liver and remained unchanged after the treatment. Kinetic studies showed that iNOS mRNA increased rapidly in both spleen and lung, reached a maximum 2-5 h after the treatment, and decreased thereafter. On the other hand, AS mRNA increased more slowly and reached a maximum in 6-12 h (by about 10-fold in the spleen and 2-fold in the lung). AL mRNA in the spleen and lung increased slowly and remained high up to 24 h. In immunoblot analysis, increase of iNOS protein was evident in the lung, liver, and spleen, and there was an increase of AS protein in the lung and spleen. In immunohistochemical analysis, macrophages in the spleen that were negative for iNOS and AS before LPS treatment were strongly positive for both iNOS and AS after this treatment. As iNOS, AS, and AL were coinduced in rat tissues and cells, citrulline-arginine recycling seems to be important in NO synthesis under the conditions of stimulation.