Perfusion fixation can redilate the fetal capillary network of the freshly delivered human placenta to its in vivo dimensions. As part of the fixation procedure in earlier experiments a venous back-pressure of 20 mmHg was applied on the rationale that this was necessary for full distension of the fetal capillaries. In order to verify this assertion, five mature placentae were fixed by perfusion fixation but without the application of such a back-pressure. No statistically significant difference was found between the values for the various parameters measured in placentae fixed in this manner compared with those processed using the original technique. In view of this finding, and of the implication that impedence to venous flow may cause fetomaternal fluid shift during in vitro perfusion experiments, it would appear that venous back-pressure should best be omitted from the perfusion fixation technique. The shrinkage rate associated with this technique was found to be approximately 2.0 per cent, based on the measurement of maternal erythrocyte diameters.