We report here quantitative data on the Feyrter (single) cells (APUD cells) and neuroepithelial bodies (grouped Feyrter cells), in the lungs of rabbit fetuses at 26, 27.5 and 29 days gestational age, during normoxia and short term chronic hypoxia. The apparent number of these cells declines during this period; we suggest that this might be due to increased hypoxemia. Moreover, the number of cells in the lungs of fetuses from short term chronically hypoxic mothers is lower than in the normoxic animals. These findings are in agreement with our previous studies in short term chronically hypoxic neonatal rabbits, and suggest that the increased hypoxemia in the fetus, caused by the induction of hypoxia in the mother, constitutes a stimulus for secretory activity of the Feyrter cells and neuroepithelial bodies (NEBs). This in turn could be part of the mechanism responsible for maintaining the pulmonary vasoconstriction due to hypoxemia. Our results from fetuses of normoxic does provide base line data on the chronological importance of the Feyrter cells and NEBs.