The selective GABA(C) receptor antagonist, (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA), is eight times more potent against human recombinant p receptors than p2 receptors expressed in Xenopus oocytes. (3-Aminopropyl)methylphosphinic acid (CGP35024), the methylphosphinic acid analogue of GABA, and [(E)-3-aminopropen-1-yl]methylphosphinic acid (CGP44530), an open chain analogue of TPMPA, were five and four times, respectively, more potent as antagonists of p1 receptors than as antagonists of p2 receptors. Isoguvacine was a weak partial agonist at both p1 and p2 receptors with intrinsic activities (calculated as a percentage of the maximum whole cell current produced by a maximum dose of GABA) of 45 and 68%, respectively, of the maximum response produced by GABA. In agreement with other workers, it was found that imidazole-4-acetic acid was a partial agonist at both p1 and p2 receptors, showing higher intrinsic activity at p2 than at p1 receptors. The p1 receptor antagonist, trans-4-amino-2-methylbut-2-enoic acid (2-MeTACA), was a partial agonist at p2 receptors with an intrinsic activity of 34%. 2-MeTACA may be useful in differentiating between homo-oligomeric p1 and p2 receptors in native systems. These studies reveal significant differences in the antagonist profile of human recombinant p1 and p2 GABA(C) receptors.