The purpose of this study was to determine whether carbopol polymers, polyacrylic acid polymers, can inhibit lumenal degradation of insulin, calcitonin and insulin-like growth factor I (IGF-I) by trypsin and chymotrypsin and to understand whether reducing the pH of the incubation medium by these polymers results in inhibition. Further, the effects of carbopol polymers on the in-situ absorption of insulin were studied in rats. In saline, carbopol polymers at 1% and 4% (w/v%) inhibited close to 100% of trypsin and chymotrypsin activities against insulin. In 50 mM Tris buffer, carbopol polymers, including 934P, 974P and 971P, at 0.1% only weakly inhibited degradation of calcitonin and insulin by both enzymes; however, as the polymer concentration increased to 0.4%, degradation of insulin, calcitonin, and IGF-I by both enzymes was complete or almost complete. When the Tris buffer was increased to 100 mM, no inhibition was observed at 0.1%. Determination of the final pH of the incubation medium in the presence of polymers revealed that the inhibitory effects of carbopol polymers correlated with the final pH. When the incubation medium has no or low buffer capacity to buffer the protons released by carbopol polymers, these polymers are able to reduce the pH much lower than the optimum pH for the enzyme activities, and thus inhibit proteolytic degradation. When the buffer capacity of the incubation medium increases, the inhibitory effects of carbopol polymers weaken. In-situ absorption of insulin revealed that carbopol polymers improved insulin absorption and induced a significantly greater decline in blood glucose levels. It is concluded that carbopol polymers with strong bioadhesive properties also can inhibit lumenal degradation of peptide hormones, offering multiple advantages for their uses in oral drug delivery.