The glutamate analogue 2-amino-4-phosphonobutyrate (APB) is known to activate a subtype of metabotropic glutamate receptor in the central nervous system, including the retina. In the present study, APB receptors were studied using the Xenopus oocyte expression system. No endogenous APB sensitivity was detected in control oocytes. In contrast, microinjection of mRNA, extracted from quail brain, into Xenopus oocytes resulted in the functional expression of APB receptors after 3-5 days incubation. Application of 50 microM-1 mM APB to injected oocytes voltage clamped at a holding potential of -60 mV produced a sustained outward current which was associated with a significant decrease in membrane conductance; the reversal potential was around -11 mV. The response to APB was dose-dependent and non-desensitizing. This is the first demonstration of the expression of a conductance-decreasing receptor mechanism in Xenopus oocytes.