1. Whole kidney and micropuncture techniques were employed to investigate the determinants of glomerular ultrafiltration in virgin and 12-day pregnant rats. 2. A significant increase in whole kidney glomerular filtration rate (g.f.r.) and superficial cortical single nephron g.f.r. was noted in pregnant rats compared to virgins. 3. Increases in whole kidney and glomerular plasma flow rate also occurred in pregnancy which were in proportion to the increase in rate of filtration. No differences were noted in the hydrostatic and oncotic pressures which influence formation of glomerular ultrafiltrate in the superficial nephron population. 4. Reduction in arterial haematocrit and no change in mean red cell volume indicate that a plasma volume expansion has occurred by day 12 of pregnancy in the rat. 5. It is concluded that the increased g.f.r. seen in 12-day pregnant rats is exclusively the result of an increase in renal plasma flow rate (r.p.f.) since the other determinants of glomerular ultrafiltration are unaffected by pregnancy. The plasma volume expansion which also occurs must be, at least in part, responsible for the increase in r.p.f.