In this study, using an enzymatic HPLC method in combination with D-aspartate oxidase, we show that N-methyl-D-aspartate (NMDA) is present at nanomolar levels in rat nervous system and endocrine glands as a natural compound, and it is biosynthesized in vivo and in vitro. D-aspartate (D-Asp) is its natural precursor and also occurs as an endogenous compound. Among the endocrine glands, the highest quantities of D-Asp (78 +/- 12 nmol/g) and NMDA (8.4 +/- 1.2 nmol/g) occur in the adenohypophysis, whereas the hypothalamus represents the area of the nervous system where these amino acids are most abundant (55 +/- 9 and 5.6 +/- 1.1 nmol/g for D-Asp and NMDA, respectively). When D-Asp is administered to rats by ip injection, there is a significant uptake of D-Asp into the adenohypophysis and a significant increase in the concentration of NMDA in the adenohypophysis, hypothalamus and hippocampus, suggesting that D-Asp is an endogenous precursor for NMDA biosynthesis. Experiments conducted on tissue homogenates confirm that D-Asp is the precursor of the NMDA and that the enzyme catalyzing this reaction is a methyltransferase. S-adenosyl-L-methionine (SAM) is the methyl group donor. In vivo experiments consisting of ip injections of sodium D-aspartate show that this amino acid induced a significant serum PRL elevation and this effect is dose and time dependent. In vitro experiments conducted on isolated adenohypophysis or adenohypophysis coincubated with the hypothalamus, showed that the release of PRL is caused by a direct action of D-Asp on the pituitary gland and also mediated by the indirect action of NMDA on the hypothalamus. Then, the latter induces the release of a putative factor that in turn stimulates the adenohypophysis reinforcing the PRL release. In conclusion, our data suggest that D-Asp and NMDA are present endogenously in the rat and are involved in the modulation of PRL release.