This work describe a straightforward method for the isolation of clearly defined plasma membrane preparations from rat brain cortex - Percoll-purified plasma membranes, bulk of plasma membranes and low-density plasma membrane fragments. [35S]GTPγS binding experiments measured in the presence of high concentration of GDP succeeded in the resolution of the agonist-responsive component of total binding. The highest efficacy of baclofen agrees well with the role of GABA and GABA-B receptors in inhibitory signalling of the brain, the order of efficacy of different agonists of opioid receptors was DADLE > DAMGO > U-69593.