Microdialysis of marmoset (Callithrix jacchus) corpora lutea in vitro was evaluated regarding morphology, activity of 3 beta-hydroxysteroid-dehydrogenase (3 beta-HSD) and progesterone (P) secretion. Two different dialysis media were used: an unbuffered ringer solution and Krebs-Henseleit buffer gassed with carbogenium. Additionally, the effects of the luteotrophin prostaglandin E2 (PGE2) on P secretion were examined for both media. In general 3 zones of tissue according to the maintenance of cellular integrity could be identified after dialysis. Structurally intact cells were found in close vicinity to the dialysis tubing or the bathing medium after 8 h of perfusion. These 2 zones were separated by a sheet of cells which showed signs of ischemic injury and whose activity of 3-beta-HSD was reduced. During dialysis with ringer solution P release stayed constantly high for a longer period of time than with Krebs buffer. With both media PGE2 stimulated P release but could not prevent the decrease in P production during dialysis with Krebs buffer. In general profiles of baseline secretion, were more stable after treatment than for untreated corpora lutea. There, under dialysis with ringer solution, the ultrastructure of cells close to the dialysis tubing was well preserved exhibiting euchromatic nuclei, tubular sER and numerous mitochondria gathered in the perinuclear region. In contrast, with Krebs buffer heterochromatization of nuclei and vesiculation of smooth endoplasmic reticulum prevailed. After application of PGE2 no histological and ultrastructural differences could be found between tissue dialysed with ringer or Krebs buffer. In these specimens the sER of zone A cells generally appeared vesiculated. Our results indicate (1) a close structure-function relationship of luteal cells in the tested system, (2) the suitability of the system to study intra-luteal regulation and (3) the necessity to control structural integrity of the dialysed tissue.