We describe the status of vasoactive intestinal peptide (VIP) receptors in regenerating liver. VIP-stimulated adenylate cyclase activity was markedly decreased in proliferating liver 3 days after partial (70%) hepatectomy. This was associated with a reduced efficacy of VIP (53% compared with controls), with no change in the potency of the peptide (ED50 0.8 nM). In contrast, forskolin- and guanosine 5'-[beta gamma-imido]triphosphate (Gpp[NH]p)-stimulated enzyme activities were not decreased after hepatectomy. The expression of Gs protein subunits (alpha and beta) was studied by cholera toxin-catalysed ADP ribosylation of alpha s and by immunoblotting of alpha s and beta subunits. Both subunits were increased in regenerating liver, further suggesting that the decreased response to VIP was not related to a decreased expression of Gs proteins. In fact, the reduced adenylate cyclase response to VIP in regenerating liver was associated with quantitative and structural changes in VIP receptors. Equilibrium binding data obtained with 125I-VIP indicated the presence of two classes of binding sites, the Kds of which were not altered after hepatectomy. In contrast, changes in binding capacity (Bmax.) were as follows: 0.11 +/- 0.01 and 0.05 +/- 0.01 pmol/mg of protein for high-affinity sites in control and hepatectomized rats respectively; and 2.3 +/- 0.2 and 0.65 +/- 0.03 pmol/mg of protein for low-affinity sites in control and hepatectomized rats respectively. Moreover, affinity labelling experiments showed that the M(r) value of 125I-VIP-receptor complexes was higher in regenerating liver than in quiescent hepatocytes, e.g. 58,000 and 53,000 respectively. It is concluded that VIP receptors are altered in regenerating liver, resulting in a decreased response of adenylate cyclase to the neuropeptide.