These studies were designed to investigate the functional and morphologic glomerular effects of pregnancy in a model of experimentally induced glomerulonephritis (GN) in the Munich-Wistar rat. Virgin and midterm (day 12) pregnant rats were studied during the autologous phase of anti-glomerular basement membrane antibody GN and were compared with normal control virgin and pregnant rats. In this model of moderately intense GN, glomerular filtration rate (GFR) and single-nephron GFR were not different from normal controls; however, glomerular micropuncture revealed that glomerular capillary blood pressure was elevated and the glomerular capillary ultrafiltration coefficient, Kfi, was reduced in rats with GN compared with normal rats. In addition a substantial proteinuria was present, and there was morphologic evidence of a moderate and fairly uniform GN. The superimposition of pregnancy to midterm (11 to 13 days) had no effects on the proteinuria or glomerular morphology in rats with GN. Also, pregnancy did not further alter the increased glomerular blood pressure or reduced Kf that occur in this model of GN. Despite the presence of an underlying glomerular disease, the maternal kidney was able to increase renal plasma flow. When underlying mild GN is present, 11 to 13 days of pregnancy does not appear to worsen the disease in the short term. Other studies are needed to determine the long-term renal effects of repeated pregnancy when underlying GN is present.