The mural thickness of fetal stem arteries of 3rd order was assessed morphometrically in 50 placentae from each of the 'toxaemia', normal pregnancy and acute fetal distress groups. Several clinical maternal and fetal variables and the syncytial sprout proliferation of the placentae were correlated with the morphometric findings. The results show that: (1) there was a significant reduction in the ratio of lumen-to-whole-diameter of the fetal arteries in 'toxaemia' as compared with the two other groups; (2) the mean lumen-to-whole-diameter ratio also differed between regions of the placenta in all groups, the most marked reduction being in the parachorial region and the least prominent in the parabasal zone; (3) no significant differences in the mean diameter ratio were found among the three sub-groups of the toxaemic pregnancies, i.e., the preeclampsia, essential hypertension and renal disease group; and (4) there was an inverse relationship between the lumen-to-whole-diameter ratios and the syncytial sprout counts in the toxaemic group.