The pharmacological properties of morpholin-2-yl-phosphinic acids were evaluated on GABA(B) receptors. In rat neocortical slices maintained in Mg2+-free Krebs medium, baclofen, a GABA(B) receptor agonist, produced a concentration-dependent depression of the frequency of spontaneous discharges with an EC50 of 14 +/- 5.5 microM, which was antagonised reversibly by the morpholin-2-yl-phosphinic derivatives. The order of potency was 3-[(3S,6R)-6-[(cyclohexylmethyl)hydroxyphosphinoylmethyl- morpholin-3-yl]benzoic acid (CGP 76290A) (pA2 = 7.1 +/- 0.05) > its enantiomer 3-[(3R,6S)-6-[(cyclohexylmethyl)hydroxyphosphinoylmethyl]-++ +morpholin-3-yl]benzoic acid (CGP 76291A) (pA2 = 6.8 +/- 0.1) > cyclohexylmethyl-[(2R',5S')-5-(3-nitrophenyl)-morpholin-2-++ +ylmethyl]phosphinic acid (CGP 71978) (pA2 = 6.5 +/- 0.05) > cyclohexylmethyl-[(2R,5S)-5-phenyl-morpholin-2-ylmethyl++ +]phosphinic acid (CGP 71980) (pA2 = 6.3 +/- 0.15) > its enantiomer cyclohexylmethyl-[(2S,5R)-5-phenyl-morpholin-2-ylmethyl++ +]phosphinic acid (CGP 71979) (pA2 = 5.8 +/- 0.1). An open chain analogue of CGP 76290A, CGP 56999A (3-[1(R)-[(3-cyclohexylmethyl-hydroxyphosphinoyl)-2(S)-hydro xypropyl-amino]-ethyl]benzoic acid lithium salt) gave a pA2 of 6.6 +/- 0.2. In GABA(B) receptor binding assays, CGP 71982 (the racemic mixture of CGP 76290A and CGP 76291A), CGP 76290A, CGP 76291A, CGP 71978, CGP 71980 and CGP 71979 had IC50 values against [3H]CGP 27492 binding of 8, 1.85, 69, 124, 326 and 1460 nM, respectively. In electrically-evoked [3H]GABA release from rat cortical slices, CGP 71982, CGP 71978, CGP 71980 and its enantiomer CGP 71979, antagonised GABA(B) autoreceptors with EC150 values of 2.5, 33, 181 and 474 nM, respectively. These compounds form a novel class of potent GABA(B) receptor antagonists.