The morphology of two cases of prolactin (PRL) secreting pituitary tumours was analyzed at the light and electron microscopic level. Prior to surgery both patients had been treated with bromocriptine. In both tumours, groups of cells revealed considerable morphological changes but these were not confined to the whole adenoma. In fact, most cells still appeared ultrastructurally intact. Pieces from both adenomas were explanted to anin vitrosystem. These specimens secreted mainly PRL though smaller amounts of growth hormone (GH) were also produced. The cell morphology of the cultured specimens often showed a large number of preserved cells.