The gross and microscopic effects of exposing placenta and umbilical cord to meconium were studied in vitro. Gross staining begins with one hour and is a surface phenomenon proportional to length of exposure and meconium concentration. Pigment penetration into the membranes with subsequent uptake by macrophages is dependent primarily on exposure length. Eight of 11 placentas exposed to various meconium solutions were found to have meconium pigment- laden macrophages in the amnion after one hour. These were seen in the chorion somewhat later. Although only three of 11 placentas showed chorionic pigmented macrophages at one hour, 12 of 12 exhibited this finding after three hours. Amniotic epithelial degeneration also showed a time correlation. These findings, especially the microscopic changes described, permit more accurate determination of the defecation-to-birth interval.