The results of light and electron microscopic studies demonstrated that Trypanosoma brucei infection produced marked testicular degeneration in deer mice (Peromyscus maniculatus) with highly significant (p less than 0.001) decreases in testicular weight, seminiferous tubular diameter, and epithelial thickness. Lesions in seminiferous tubules consisted of necrosis of spermatids, spermatocytes, and sometimes the spermatogonia. Sertoli's cells had marked vacuolation and accumulation of phagocytic material. Orchitis was present in most infected mice, particularly those with severe seminiferous tubular degeneration. There was marked folding of the basal laminae of the seminiferous tubule and myoid layer, and a consistent increase in the layers of the basal laminae of the seminiferous tubule. Trypanosomes were present in the intertubular tissues of seminiferous tubules, and they occasionally crossed the myoid layer but never the basal lamina of seminiferous tubules. Inflammatory and other cells in the interstitium consisted of lymphocytes (35.2%), Leydig's cells (25.1%), macrophages (18.1%), plasma cells (12.3%), neutrophils (5.6%), eosinophils (3.2%), and mast cells (0.5%). Inflammatory cells including lymphocytes (51.6%), macrophages (43.9%), plasma cells (3.0%), and eosinophils (1.5%) occasionally breached the myoid layer and occupied the space between it and the basal lamina of the seminiferous tubule. Leydig's cells usually existed in clusters, had decreased mitochondrial size and secretory granules, and folding of the nuclear membrane.