A close relationship was assumed between the developments of nephropathy and thymoma in the previous study, in which the effect of introduction of the rat nude gene was studied in high thymoma BUF/Mna rats. In this paper, the effect of neonatal thymectomy on the development of nephropathy was examined to clarify the relationship between these two lesions in BUF/Mna rats. The average amount of urinary protein excreted from sham-operated and thymectomized BUF/Mna rats was 30.8 +/- 17.1 and 40.0 +/- 20.0 mg/day, respectively, and the number of affected glomeruli per 100 glomeruli 4.9 +/- 1.0 and 5.3 +/- 2.0, respectively. There were no significant differences in the urinary protein content, the number of the affected glomeruli, and immunofluorescence findings. In a control group, ACI/NMs rats exhibited 8.9 +/- 2.6 mg/day protein in urine and 0.8 +/- 0.4% affected glomeruli, which were significantly different from that of sham-operated and thymectomized BUF/Mna rats. These results indicate that nephropathy in BUF/Mna rats results neither from thymoma itself nor from the immunological abnormality secondary to it, and suggest that this lesion might be ascribed to a genetic factors.