Morphometrical relations of human placental villi from 14 normal placentas of the 38th to 42nd week of pregnancy terminated by Cesarean section were examined in relation to the method of fixation. Villous tissue was obtained from central areas of the in situ placenta with intact maternal circulation by means of puncture aspiration and punch biopsy. Further punch biopsies were performed 2, 5, 10 and 20 min after manual removal of the placenta. Semithin sections of these villi were photographed and examined morphometrically. The findings demonstrate clear effects on quantitative data as a result of the period of ischemia between manual removal and fixation of tissue. Significant changes of several parameters (mean trophoblastic volume, size of epithelial plates, volume of connective tissue, volume of fetal vessel lumina, etc.) occur as early as 2 min after separation of the placenta from the maternal circulation. They are mostly related to a collapse of intravillous vessels. The villous branches may be classified according to structure and morphometry, if they have been obtained under optimal conditions of fixation and prior to any vascular collapse.