Little is known about the mechanism of action of anesthetics at the biochemical level. The present work, however, gives evidence that barbiturates inhibit inositol phospholipid hydrolysis in both intact and permeabilized rat basophilic leukemia (RBL-2H3) cells by an effect on GTP-binding proteins (G-proteins). Inhibition of hydrolysis was observed when intact cells were stimulated with antigen (DNP24BSA) or with oligomers of IgE. The inhibition was dependent on the concentration and type of barbiturate used with an order of inhibitory action of secobarbital < S(-) pentobarbital < pentobarbital < R(+) pentobarbital < phenobarbital. The relatively inactive analogue, (l'RS, 3'SR) 3-hydroxypentobarbital caused little (<30% at 1 mM) or no inhibition (at 0.1–0.5 mM). In permeabilized cells, the hydrolysis induced by DNP24BSA and the nonhydrolyzable analogue of GTP, GTPγS (2–100 MM), was also inhibited by pentobarbital. The inhibition of hydrolysis was decreased as pH increased, and was no longer apparent at pH 7.8, a possible indication that the inhibitory effect was due to the unionized form of the drug. In permeabilized cells, the inhibition by pentobarbital occurred in the presence or absence of Ca2+and was uncompetitive in nature (Km= 7.1 μM for GTP in controls vs. 1.6 μM in the presence of 0.5 μM pentobarbital). Taken together, the data suggest that barbiturates alter the activity of G-proteins independently of Ca2+, and the inhibition may depend on both the hydrophobic properties and the stereospecific and structural features of the molecule.