Excitatory amino acid stimulation of phosphatidylinositol (PI) hydrolysis has been associated with development of the CNS. Normally minimally ineffective in stimulating PI hydrolysis in the neonatal rat cerebellum, N- methyl- d-aspartate (NMDA) increased levels of PI hydrolysis 82.3 ± 5.5% above basal values in the presence of 1 μM baclofen, a γ-aminobutyric acid B (GABA B) receptor agonist. This effect was observed at day 7 but not in adult cerebellum. The effect of baclofen could be mimicked by low dose GABA and taurine, actions which were blocked by prior application of a specific GABA B antagonist. Therefore, the ability of NMDA to stimulate PI hydrolysis in neonatal cerebellar tissue may be regulated by the degree of GABA B receptor stimulation.