Propofol is a sedative and anesthetic agent that can both activate GABAA receptors and potentiate receptor activation elicited by submaximal concentrations of the transmitter. A recent modeling study of the β3 homomeric GABAA receptor postulated a high-affinity propofol binding site in a hydrophobic pocket in the middle of a triangular cleft lined by the M1 and M2 membrane-spanning domains of one subunit and the M2 domain of the neighboring subunit. The goal of the present study was to gain functional evidence for the involvement of this pocket in the actions of propofol. Human b3 and a1b3 receptors were expressed in Xenopus oocytes, and the effects of substitutions of selected residues were probed on channel activation by propofol and pentobarbital. The data demonstrate the vital role of the β 3(Y143), β 3(F221), β 3(Q224), and β 3(T266) residues in the actions of propofol but not pentobarbital in β 3 receptors. The effects of β 3(Y143W) and β 3(Q224W) on activation by propofol are likely steric because propofol analogs with less bulky ortho substituents activated both wild-type and mutant receptors. The T266W mutation removed activation by propofol in β 3 homomeric receptors; however, this mutation alone or in combination with a homologous mutation (I271W) in the α1 subunit had almost no effect on activation properties in a1b3 heteromeric receptors. We hypothesize that heteromeric α1β3 receptors can be activated by propofol interactions with β 3- β 3, α1- β 3, and β 3- α 1 interfaces, but the exact locations of the binding site and/ or nature of interactions vary in different classes of interfaces.