The functional capacity of the placenta of the guinea pig has been reduced in four different ways. It has been investigated whether changes in the trophoblast of the labyrinthine part of the placenta occurred which could be interpreted as a compensation mechanism for the reduction of the capacity. No significant decrease of the thickness of the maternofetal barrier could be measured nor did enlargement by microvilli of the apical and basal surface of the syncytium change significantly. Activities of the enzymes glucose-6-phosphate dehydrogenase, lactate dehydrogenase, glucose-6-phosphatase and adenosine triphosphatase showed a great variation, but no differences in activities could be demonstrated. Further, the number of cytotrophoblastic cells was widely spread and no significant difference could be observed, although sometimes large and apparently newly formed parts of lobulus were observed.