[en] Vasoactive Intestinal Peptide (VIP) exerts multiple biologic actions through binding to specific receptors. The authors have solubilized VIP-receptors from guinea pig lung by the detergent CHAPS and separated the soluble receptors by high performance gel-filtration into two fractions. The binding of [¹²⁵I]-VIP to the two fractions was time-dependent, reversible and saturable. Kd for the 6.2 nm- and 2.3 nm-fractions were, respectively, 0.3 nM and 0.9 μM. GTP inhibited VIP-binding to the 6.2 nm-, but not the 2.3 nm-species. The GTP-treated 6.2 nm-fraction showed two classes of binding sites with lower affinity (Kd: 4.4 nM and 153 nM). In increased detergent concentration (9 mM), 46% of the binding-activity of the 6.2 nm species appeared in the 2.3 nm-fraction. The homologous peptides PHI and GRF, were, respectively, 87.5-fold and 22.9-fold less reactive than VIP with the 6.2 nm-complex. GRF was 22.7-fold more reactive and PHI was 31.1-fold less reactive than VIP with the 2.3 nm-species. The binding by the 6.2 nm-fraction was decreased by phospholipases A₂ (73%) and C (58%), but both enzymes increased binding by the 2.3 nm fraction (61-106%). In conclusion: (1) GTP-binding regulatory proteins are functionally coupled to the VIP-binding subunit in the 6.2 nm fraction, (2) the 2.3 nm-fraction is a dissociation product of the 6.2 nm-complex, (3) the change in the relative reactivity of VIP and GRF with the two receptor fractions may reflect differential regulation of receptor affinity by G-proteins and (4) bound phospholipids may modulate VIP receptor-number and affinity