The effects of trypsin treatment on VIP binding to rat intestinal epithelial cell membranes were examined. The decrease in specific binding of [125I]VIP is dependent on the amount of trypsin used and digestion time. Specific binding decreases by 50% after 8 min with 20 micrograms/ml trypsin. Trypsin is active in the 1-100 micrograms/ml concentration range (ED50 approximately equal to 5 micrograms/ml). Non-specific binding is unaltered by the enzyme. The effect of trypsin is abolished by trypsin inhibitor. Scatchard analysis of VIP binding reveals two types of binding sites: sites I characterized by a high affinity, a low capacity and a high sensitivity to low trypsin levels (1-5 micrograms/ml); sites II characterized by a low affinity, a high capacity, resistant to low trypsin levels (1-5 micrograms/ml) but sensitive to a high trypsin level (20 micrograms/ml). Trypsin decreases the binding capacity by lowering the site number without altering their affinity. Sites not destroyed by trypsin retain their functional characteristics: KD, sensitivity to GTP and coupling with adenylate cyclase. It is concluded that sites I and II are proteins with different structures and/or differently localized in the membrane.