The nonobese diabetic (NOD) mouse is a model for human Type 1 diabetes mellitus. Pancreaticβ-cell destruction in NOD mice is mediated by an autoimmune process which can be accelerated by cyclophosphamide (CP). We studied the phenotype of lymphocytes from central, peripheral and regional lymphoid tissues in prediabetic NOD and C3H mice before and after a single large dose of CP. All lymphoid organs showed a greatly diminished cell number and most alterations appeared early after CP and were transient, but an aggressive insulitis was not seen in NOD mice until 14 d after injection. The pancreatic islets in C3H mice remained intact and were not infiltrated. NOD female mice, which are most prone to spontaneous and CP-induced diabetes, exhibited the most unusual lymphoid kinetics after treatment with CP. Their thymus and spleen showed the least relative drop in total cell number and the most rapid rate of recovery. The thymus of these mice was also found to have an increased proportion of CD3+thymocytes while CD4/CD8 double positive thymocytes decreased 7 d after CP. At 14 d after CP the number of IL-2R+thymocytes had surpassed that of normal levels. The most dramatic observation was the rapid recovery and overshoot in the number of pancreatic lymph node cells of female NOD mice which coincided with aggressive insulitis.