The hypoglycemic effect of orally given insulin was studied on rabbits, using two different absorption promoters, and two different carriers. The effects of hydroxypropyl cellulose (HPC) of different degrees of substitutions (Klucel R GF, MF, and HF), and consequently of different viscosities, on the oral bioavailability of insulin have been studied. The fatty acid-Brij R system was not affected by the HPC studied, while the salicylate-cellulose system resulted in a significantly better absorption of insulin in the presence of the medium viscosity Klucel R . More than 70% reduction in blood glucose levels was achieved for rabbits, treated with the salicylic acid-Klucel MF-Avicel R system containing 5 units of insulin per kg of animal weight.