Summary Higher amounts of plant growth substances such as, indole acetic acid (IAA), cytokinin-like substances (CK), gibberellic acid-like substances (GA), abscisic acid-like substance (ABA) were obtained from mature root nodules of the leguminous tree, Pterocarpus mursupium as compared with young or old ones. Higher levels of nitrogenase (N 2 -ase) activity together with IAA metabolism as evaluated by levels of indole acetic acid oxidase (IAA oxidase), methyleneoxindole reductase (MeOx reductase), phenol, peroxidase, and polyphenol oxidase in mature nodules indicated higher metabolic efficiency of mature nodules than in young or old ones. In mature nodules IAA and CK together with N 2 -ase were highest in the rainy season, whereas, GA and ABA were highest in winter. N 2 -ase also was lowest in winter. Thus, N 2 -ase may have positive relations with IAA and CK and negative relations with GA and ABA. Metabolism of IAA, was found to change inversely with the changes of IAA oxidase activity which was again regulated by changes of phenol levels. The phenol level, in turn, was regulated by the activities of peroxidase and polyphenoloxidase in the nodules. Tryptophan (Trp) level showed a positive relation with the IAA level in the nodules. In spite of the variation in content of the hormones with seasons, the size of the nodules was fixed at a definite age in all the seasons indicating that hormone contents were not solely related to the development of nodules. The roots of the plant did not show any N 2 -ase activity and always had lower levels of hormones. The IAA metabolic rate was much more pronounced in roots than in nodules which may be the cause of the lower IAA level in them.