A morphological study has been undertaken on the capillaries of 9 microprolactinomas and 9 macroprolactinomas, surgically removed from untreated patients. The study was carried out utilizing light and electron microscopic techniques and electron microscopic morphometry. The frequency of the capillaries and their structural appearance were taken into account. The frequency of capillaries was found to be very different in micro- and macroadenomas. In microadenomas 51.1 capillaries/0.1 mm2 of tissue section were observed; this value was not significantly different from that found in normal human pituitaries (62.0/0.1 mm2). In contrast, in macroprolactinomas a much lower degree of vascularization was found (9.3 capillaries/0.1 mm2 of tissue section). The capillary abnormalities previously reported for pituitary adenomas (endothelial thickening, swelling and blebbing, loss of fenestration, multilayered basal membrane, etc.) were observed in all prolactinomas studied, but no differences were found between the two types of tumors. In both types of tumors, the capillaries generally looked mature. Very rare sprouting capillaries were observed. Angiogenesis is likely to be slow, in agreement with the low frequency of capillaries in the more rapidly proliferating tumors such as macroprolactinomas. The different frequency of capillaries in micro- and macroprolactinomas could have some important consequences as to the regulation of the hormonal secretion. In fact, the different blood supply to the small and large tumors could result in a different availability of regulatory factors for the two types of tumors.