The superficial morphology of the pleura has been observed in humans by scanning electron microscopy (SEM). Pleural samples from the visceral, mediastinal, diaphragmatic and costal normal human pleura were observed, and a thorough morphometric study was performed. The most evident feature was the ubiquitous presence of microvilli and micropores. The secretion and absorption activities therefore should not be regarded as accomplished by different topographic zones of the pleura. Discontinuities and clefts were observed at the level of cellular junctions; absorption of high weight molecules and cells from the pleural space may be facilitated at this level, but the structures which were observed in humans may not be considered "stomata," according to the definition of such morphologic units as we give in the present study. Cilia and blebs, described in the experimental animal or in other mesothelia, were not seen in the human pleura.