1. Serum albumin, total protein, oncotic pressure and osmolality were measured in a group of patients with hyperprolactinaemia due to a prolactin secreting tumour, normal subjects and normoprolactinaemic acromegalic patients.
2. A significant increase in albumin, total protein and oncotic pressure was found in the prolactinoma patients when compared with those of the other two groups.
3. There was no difference in serum osmolality between the three groups.
4. These studies provide further evidence that prolactin may have anabolic effects.