Phosphoribosyl pyrophosphate (PPRibP), required in nucleotide synthesis, increases 2-fold in rat kidney from 1 day post partum to adult stage; there is no accompanying increase in PPRibP synthetase activity measured in vitro. Ribose 5-phosphate is a key factor in the regulation of PPRibP synthesis. The activity and regulation of 3 routes of ribose 5-phosphate formation have been measured in renal growth:(i) the flux through the oxidative pentose phosphate pathway was high in the neonatal period but increased only+ 50% thereafter;(ii) the non-oxidative pentose phosphate pathway, including transketolase, increased by+ 145%;(iii) the rate-limiting enzymes of the glucuronate-xylulose route increased+ 200% from 1 day to the adult stage. The importance of systems reoxidizing NADPH was shown by:(i) the stimulation of renal PPRibP formation from glucose by phenazine methosulphate;(ii) the early involvement of the oxidative pentose phosphate pathway at the stage where NADPH is used for biosynthetic routes;(iii) the increasing involvement of the glucuronate-xylulose route, which acts as a transhydrogenase producing NADP+ in addition to pentose phosphate formation and (iv) the correlation between renal PPRibP content and the activity of aldose reductase, which, by utilization of NADPH, stimulates ribose 5-phosphate formation via the oxidative pentose phosphate pathway. Evidence is adduced that the contribution of the 3 routes of ribose 5-phosphate formation in the kidney varies at different stages of development.