Three hundred day-old Japanese quail (Coturnix coturnix japonica) were divided into two groups with 150 quail in each group. One group was maintained on quail mash alone, while Fusarium moniliforme culture material was added to quail mash in the second group from day 5 of age and was supplied at a rate of 150 ppm fumonisin B1 (FB1)/kg mash. At day 21, each group was further subdivided into two groups, yielding four groups with 75 birds apiece, which served as the control (group CX), the Salmonella Gallinarum alone group (group CS), the FB1 alone group (group FX), and the group fed FB1 and infected with Salmonella Gallinarum (group FS). An oral challenge with Salmonella Gallinarum organisms (2 x 10(4) colony-forming units/ml) was given to groups CS and FS at 21 days of age. Three quail each were necropsied on day 21 (0 day interval) from groups CX and FX only. At subsequent intervals (i.e., 1, 2, 3, 5, 7, 10, 14, and 21 days postinfection [DPI]), three quail were euthanatized from all four groups (CX, CS, FX, and FS). The gross and microscopic lesions were recorded in both mortality and euthanatized birds at the above intervals. The ultrastructural studies were done at 5 DPI. Mild to moderate hepatomegaly and pale discoloration of liver were observed in group FX, while congestion, hemorrhages, necrosis, and mild to severe hepatomegaly were the predominant gross lesions in both infected groups (CS and FS). The gross lesions in quail inoculated with Salmonella Gallinarum alone (group CS) generally developed slowly, appeared more widely scattered, and involved comparatively less surface area in contrast to the rapidly progressive and frequently confluent lesions in the combination group (FS), especially in the first 5 days of infection. Mild to marked hepatocellular swelling, multifocal hepatic necrosis, and hepatocellular and bile duct hyperplasia were the characteristic microscopic changes in the FX group. Microscopic lesions in quail of group CS comprised congestion, vacuolar changes, and focal necrosis in early stages, followed by granulomatous lesions at later intervals. Similar but more severe lesions were observed in the combination group (FS). Based on transmission electron microscopy, the maximum effect of FB1 toxicity was observed on mitochondria and endoplasmic reticulum. In general, the mitochondriae showed diverse form and structure, some of which appeared to lose their intact outer membrane, and the mitochondrial cristae were disoriented. The deformity in the cisternae structure of rough endoplasmic reticulum, with their rearrangement into round or tubular forms either bearing granular surface or leading to accumulation of smooth endoplasmic reticulum, was evident only in groups FX and FS. We conclude that the continuous presence of fumonisins in the diets of young quail might increase their susceptibility to or the severity of Salmonella Gallinarum infection.