A combination of thymectomy and sublethal irradiation (Tx-X) consistently induced diabetes in female rats of the PVG/c strain. The incidence of diabetes varied from 10.7% to 53.4% in seven successive Tx-X groups (mean 29.7%). Both clinical and subclinical disease was observed with the majority of affected animals developing the former condition. This was acute in onset, rapidly fatal (1–4 days) and characterised by ketosis and lipidaemia. Overtly diabetic rats had markedly raised plasma glucose concentrations compared to normal rats of the same strain and plasma immunoreactive insulin concentrations were comspondingly depressed in this group.Histopathological change within the islets of Langehans correlated with clinical status and ranged from diffuse atrophy in the majority of the acutely diabetic rats to mild and focal lymphocytic insulitis in a proportion of the non-diabetic rats.Islet cell autoantibodies were demonstrated by indirect immunofluorescence in approximately 25% of clinically diabetic animals.The majority of diabetic rats were found to be responsive to insulin and the clinical signs could be reversed by daily parented insulin administration.These observations implicate the immune system in diabetes generation and are consistent with an immune mediated pathogenesis as the underlying cause of the islet cell destruction. This syndrome may thus be a potentially useful animal model for type 1 (insulin dependent) diabetes in man.