Mycobacterium avium subsp. silvaticum (Mas) has been observed to produce intestinal lesions in ruminants, such as calves or deer, resembling those of Paratuberculosis, and suggesting that Mas could be involved in this disease of ruminants. In order to study the lesional aspects of the infection with Mas and Mycobacterium avium subsp. paratuberculosis (Map) and to establish possible differences or similarities between both infections, an experimental infection with Mas and Map in two lamb groups was carried out. The lambs were killed at different times: 60, 120, 220, 500 and 650 d.p.i. being the digestive system, especially the gut associated lymphoid tissue, studied in detail. Gross and microscopic lesions were clearly different in both infections, the animals infected with Mas showing typical lesions of intestinal tuberculosis, located only in the lymphoid tissue (jejunal and ileal Peyer's patches and lymph nodes); these lesions showed a clear tendency to regression with time. However, the lambs infected with Map showed focal lesions (microscopically different from those produced by Mas), only located in Peyer's patches, more in jejunal than in ileal, that progressively involved the mucosa and the mesenteric lymph nodes. In some lambs, a diffuse granulomatous enteritis was seen at the last slaughter. In conclusion, the type of lesions and the evolution in the time were different in both infections.