The aim of this study was to determine the glomerular filtration rate (GFR) in the three trimesters of pregnancy, using creatinine and urea clearances. A total of 108 healthy subjects between the ages of 18 and 37 years were divided into non-pregnant (28 ± 5 years, n = 30), 1st trimester of pregnant (28 ± 5 years, n = 18), 2nd trimester of pregnant (28 ± 4 years, n = 30), and 3rd trimester of pregnant (30 ± 4 years, n = 30) women. Serum and urine samples were collected in mid-trimester period. Creatinine and urea concentrations in serum and urine were determined using Jaffe’s method (Bosnes et al. 1945) for creatinine, and the urease method (Martinek, 1964) for urea. There was a significant increase (mean ± SD) in creatinine clearance (Ccr) when compared with the non-pregnant value. The increases of 43 ± 26 ml/min (non-pregnant vs. 1st trimester), of 39 ± 21 ml/min (1st vs. 2nd trimesters) and of 42 ± 25 ml/min (2nd vs. 3rd trimesters) were similar. These increases in Ccr were associated with a significant fall (p<0.001) in serum creatinine concentration. Similarly, there was a significant increase in the Urea clearances in the 1st, 2nd and 3rd trimesters of pregnancy, and remained elevated during pregnancy. In contrast, blood urea concentration increased significantly (p<0.01) in the three trimesters, despite an increase in the 12 hour urine volume output. The results in Table 1 show that Ccr and Cur increase and peak during the first trimester of pregnancy. Although there was a reduction in Ccr in the 3rd trimester compared to the 2nd trimester, the clearance remained significantly higher (p<0.05) than in the non-pregnant women. This study agrees with other workers (Logoglu et al. 1990; Susan & Donna, 1992), who showed similar increases in GFR during pregnancy. It is concluded that GFR increases and peaks during the first trimester of pregnancy and remains relatively stable till term.