The effect of guanine nucleotides and related analogues on the binding of 2-[<sup>125</sup>I]-melatonin to membranes prepared from ovine pars tuberalis was studied. Dose-dependent inhibition of 2-[<sup>125</sup>I]-melatonin binding was observed, with an order of potency of GTPγS > Gpp(NH)p > GTP = GDP. GMP, cyclic GMP and ATP had negligible effects. Analysis of saturable binding revealed that GTPγS (1 µ M) promoted an apparent reduction in receptor density of about 50%, without a concomitant change in receptor affinity. These results are consistent with a melatonin receptor existing in an equilibrium between high- and low-affinity states, with GTP and related analogues able to cause a shift in the equilibrium in favour of the lower-affinity form. The sensitivity of 2-[<sup>125</sup>I]-melatonin binding to guanine nucleotides implies the presence of a melatonin receptor on the ovine pars tuberalis, the action of which is mediated via a G protein.