We have recently reported evidence that a simple β-linked alkylated mannose reversibly increased the magnitude of GABA A receptor currents evoked in cultured rat pyramidal neurons whilst concomitantly reducing the incidence of spontaneous synaptic activity. In this present study, the effects of the simple β-linked disaccharide, lactose was investigated using a [ 3 H ] TBOB ( t-[ 3 H ] butylbicycloorthobenzoate) binding assay in adult rat forebrain and cerebellum membranes. Lactose elicited a significant potentiation of [ 3 H ] TBOB binding to well-washed forebrain and cerebellar membranes (mean E max values=367 and 287%; mean ec 50 values=1.5 and 30 μM, respectively, N=4). The α-linked disaccharides, maltose and sucrose also potentiated [ 3 H ] TBOB binding, but with 100–600-fold higher ec 50 values than lactose. The lactose-mediated potentiation of [ 3 H ] TBOB in the forebrain and cerebellum was completely abolished in the presence of 0.3 μM GABA. Over the concentration range in which significant potentiation of [ 3 H ] TBOB binding was detected, lactose elicited no significant effect upon [ 3 H ] flunitrazepam binding. This study demonstrated that lactose can modulate the GABA A receptor channel, allosterically coupled to the agonist site, but independent of the benzodiazepine site. Furthermore, lactose displayed differential effects upon forebrain and cerebellar GABA A receptors indicating that it may be a novel subtype selective agent.