The release of labeled GABA, synthesized from [(14)C]glutamine and from [(14)C]glutamate, was studied in mouse brain cortex slices. The spontaneous release of radioactive GABA derived from glutamate was higher than that of GABA synthesized from glutamine. In contrast, depolarization by 48 mM K(+) notably stimulated the release of GABA synthesized from glutamine but did not affect significantly the release of GABA formed from glutamate. It is concluded that glutamate is a better precursor than glutamine of the GABA pool spontaneously released, whereas the reverse is true for the pool releasable by K(+)-depolarization. The release of [(14)C]glutamate was stimulated by K(+) depolarization both when it was exogenously supplied and when it was derived from [(14)C]glutamine.