The VIP receptor cloned from rat lung (VIP1 receptor from the group of the PACAP-VIP type II receptors) was inserted into a mammalian expression vector and stably transfected into Chinese hamster ovary cells (CHO). Two clones were selected, expressing respectively a high (850 +/- 50 fmol/mg protein, for clone 3) and a low (100 +/- 30 fmol/mg protein for clone 16) number of receptors. Both clones had the same apparent Kd value of binding for VIP and related peptides. The receptor expressed had the same binding properties as the natural VIP receptor, judged from the relative potency of VIP and PACAP analogues and fragments. The EC50 value of adenylate cyclase activation were 3 to 10 fold lower in clone 3 than in 16. The values observed in clone 16 were closer to the binding Kd values. The differences between the two clones were explained by the existence of spare receptors in clone 3, since: (a) the relative efficacy of some fragments were lower in clone 16 than in clone 3; (b) pretreatment of the cells with VIP reduced the number of receptors in both clones and increased the EC50 value for VIP in clone 3 but decreased peptide efficacy in clone 16 without significant change of the EC50 value.