Levels of carbonic anhydrase B and C isozymes (CA B and CA C) of the hemolysates of 223 normal pregnant and 49 normal puerperal women were determined by means of single radial immunodiffusion. CA activity and progesterone in the blood of the women were also tested. The CA B level was almost the same as the control level, 13.5 +/- 1.7 mg/g Hb, in the first trimester, then slightly but significantly increased from the middle of the second trimester (p less than 0.02), reached the maximum, 17.2 +/- 2.6 mg/g Hb (p less than 0.001), in the last of the pregnancy, and reduced slightly in the puerperium. The average CA B level of a single erythrocyte significantly increased from the middle of the second trimester and amounted to 0.50 +/- 0.09 pg (p less than 0.001) in the last of the pregnancy. A significant increase of the CA C level and the CA activity was found from the last of the second trimester (p less than 0.001). The CA B level of the hemolysates of the pregnant women correlated with serum progesterone (r=0.557, p less than 0.001), and the hemolysate CA activity correlated with the CA B (r=0.766, p less than 0.001) and CA C levels (r=0.709, p less than 0.001). It seems that increased CA in pregnancy plays an important part in gaseous exchange across the placenta.