In pancreatic lesions of non-obese diabetic (NOD) mice the expression of inducible nitric oxide synthase (iNOS) and of the cytokines interferon-gamma and interleukin-4 were studied. Strong iNOS expression as determined at the level of transcription, translation and of enzyme activity was associated with destructive insulitis as seen 8-10 days after cyclophosphamide treatment of 70- to 80-day-old female NOD mice. Immunohistochemistry showed iNOS associated with infiltrating macrophages but not in endocrine cells. The enhancement of iNOS after cyclophosphamide correlated with an increase of T-helper type 1 (Th1) associated interferon-gamma expression while T-helper type 2 (Th2) associated interleukin-4 was the dominant cytokine prior to cyclophosphamide and after diabetes onset. We conclude that insulitis in young NOD mice is carried by Th2 cells while cyclophosphamide enhanced insulitis is determined by Th1 cells. Macrophages show two different functional states in insulitis; strong iNOS expression in macrophages is associated with destructive insulitis.