The developmental profiles of the neurotransmitter recognition site, labeled by [(3)H]muscimol, and of the two main modulatory sites, labeled by [(3)H]flunitrazepam and [ (35)S ]t- butylbicyclophosphorothionate , respectively, within the GABA(A) receptor complex, have been determined in chick tectal lobes between embryonic day 8 and postnatal day 20. The consonance among the rates of appearance and accumulation of the three receptor sites in tectal membranes suggests a coordinated expression and assembly of the protein subunits involved in the spatial configuration of the complex and its three binding sites, although the existence of isolated muscimol binding subunits during early embryogenesis cannot be excluded at the present time. Furthermore, the total number of binding sites of each kind, per pair of lobes, is compatible with a 1/1/I stoichiometry. The GABA(A) receptor complex reaches a maximum of expression, relative to total membrane protein, immediately after hatching, suggesting that the tectal GABAergic system may be instrumental in damping the effects of sudden exposure to light of the chick visual system upon eye opening.