Islet cell antigens have been administered orally and intravenously (I.V.) to NOD mice to assess their abilities to protect from or delay the onset of diabetes, and thereby provide insights that may have therapeutic implications in human trials. Whereas we and others have observed a delay in the onset of diabetes in NOD mice that have been fed with insulin from early life, we report here for the first time that feedings with porcine GAD65 alone (p = 0.226) or in combination with insulin (p = 0.011), have anti-diabetic effects in a prolonged study period (>400 days). While antigen-specific inhibitions of in vitro lymphocytic proliferation responses were seen (p < 0.05), antibody levels were unaffected by oral antigen treatments. IFN-gamma mRNA levels were downregulated in the islet infiltrates following oral antigen treatments while IL-2 and TNF-beta were expressed in all instances. We also observed that I.V. human recombinant GAD65, and porcine GAD given at weaning, delayed diabetes onset (p = 0.004) while similar treatments with a variety of inactive insulin preparations were generally ineffective. These findings thus indicate varying effects of oral and I.V. autoantigen administrations on the development of diabetes in NOD mice, and describe the immunological processes induced by oral autoantigen treatments.