Our data indicate the significant intrinsic efficacy of GABA(B)-receptors in rat brain cortex already at birth (PD1, PD2). Subsequently, baclofen- and SKF97541-stimulated G-protein activity, measured by agonist-stimulated, high-affinity [(35)S]GTPgammaS binding assay, was increased; the highest level of both baclofen and SKF97541-stimulated [(35)S]GTPgammaS binding was detected between PD10 and PD15. In older rats, baclofen- and SKF97541-stimulated [(35)S]GTPgammaS binding was continuously decreased so, that the level in adult, 90-days old animals, was not different from that in newborn animals. The potency of G-protein response to baclofen (characterized by EC(50) values) was also high at birth but unchanged by further postnatal development. An individual variance among different agonists was observed in this respect as the potency of SKF97541 response was decreased between the birth and adulthood. Accordingly, the highest plasma membrane density of GABA(B)-R, determined by saturation binding assay with antagonist [(3)H]CGP54626, was measured in 1-day old animals (2.27+/-0.08 pmol · mg(-1)). The further development was reflected in a decrease of [(3)H]CGP54626 binding as the B(max) values of 1.38+/-0.05 and 0.93+/-0.04 pmol · mg(-1) were determined in PM isolated from 13- and 90-days old rats, respectively.