The purpose of this work was to investigate the ability of poly(isobutylcyanoacrylate) nanocapsules to protect insulin from degradation by proteolytic enzymes providing biologically active insulin by the oral route. Insulin was labeled with Texas Red ยฎ for release studies and microscopy observations. The fluorescent marker was mostly retained by the nanocapsules incubated in the reconstituted gastric medium but the release was 75% within 30 min when the nanocapsules were incubated in the reconstituted intestinal medium. Turbidimetric measurements and electron microscopy observations confirmed that the nanocapsules were degraded in the reconstituted intestinal medium, whereas nanocapsule integrity was preserved in the reconstituted gastric medium. In vivo studies of the gastrointestinal distribution of insulin-loaded nanocapsules after oral feeding showed that nanocapsules were retained by the stomach for 30 min. One hour after oral administration, nanocapsules reached the lower part of the intestine (ileum). Fluorescence microscopy and confocal microscopy carried out on portions of the small intestine revealed the presence of concentrated fluorescent spots into the mucosa and even in the lamina propia, suggesting that insulin-loaded nanocapsules could cross the intestinal epithelium. These data suggest that PIBCA nanocapsules can efficiently protect insulin when given orally. In addition, they seemed to be significantly involved in the absorption mechanism.