The following points summarize these findings: (i) there are 2 glutamine utilizing enzyme systems in the rat kidney; (ii) the cytoplasmic glutamyltransferase system hydrolyzes either glutamine isomer while the mitochondrial localized glutaminase 1 is specific for the L-isomer; (iii) the cytoplasmic pathway contributes 70% of the total renal ammonia production in the normal kidney; (iv) chronic metabolic acidosis results in a 20-fold activation of the mitochondrial glutaminase 1 pathway.