Aims/hypothesis. Type 1 diabetes mellitus is a multifactorial autoimmune disease characterised by selective destruction of beta cells in the islets of Langerhans. We have previously shown that IL-1β modulates beta cell function, causes beta cell death and induces expression changes in 82 out of 1815 protein spots detected by two-dimensional gel electrophoresis (2-DGE) in diabetes-prone bio-breeding (BB-DP) rat islets in vitro. The aim of this study was to describe the relevance of these proteins in the development of diabetes in vivo. Methods. Syngeneic neonatal islets (n=200) were transplanted under the kidney capsule of 30-day-old BB-DP and control rats, removed to different time points after transplantation or at the onset of diabetes, and metabolically labelled with S35-methionine for 2-DGE. The 82 proteins were re-localised and followed. In addition, transplants were examined for expression of IL-1β mRNA by in situ hybridisation. Results. All 82 proteins could be re-localised in all syngeneic transplants from BB-DP and control rats. A total of 60 of the 82 proteins were changed during development of diabetes. Of the 82 proteins, 32 were changed in expression at the onset of diabetes compared to non-diabetic BB-DP rats, and 25 of these were changed as by IL-1β in vitro. Highest expression of IL-1β mRNA was found at the onset of diabetes. Conclusions/interpretation. IL-1β-induced protein expression changes in islets in vitro also occur in vivo and change in a complex pattern during the development of diabetes in the BB-DP rat. No single protein seems to be responsible for the development of diabetes, but rather the cumulative numbers of changes seem to interfere with the intracellular stability of the beta cell.