Twenty-four patients with non-secreting tumours of the hypothalamic-pituitary region, diagnosed by radiographic procedures and confirmed at surgery, were examined before and after the surgical treatment to establish a correlation between Prl responsiveness to dynamic tests and the location of pathological tissue. Three groups of patients were identified. In 10 patients with an intrasellar tumour (group I), Prl had a positive response to TRH, metoclopramide (MCP), and nomifensine (NOM), both pre- and postoperatively. In 4 patients with tumours located entirely in the hypothalamus (group II), Prl responded to TRH, but remained unresponsive to MCP and NOM. Only a partial normalization of the Prl responses was obtained after surgery. Finally, 10 patients had an intra- and extrasellar tumour (group III). In 8 of them Prl responded to TRH, but the increment was lesser than that observed in hypothalmic tumours. Prl did not respond to MCP and NOM. After surgery, Prl responsiveness reappeared in some patients (6 after TRH, 4 after MCP, 3 after NOM). In 2 patients with extensive pituitary damage, Prl did not respond to all dynamic tests pre- or postoperatively. In conclusion, Prl dynamic tests are a reliable tool for studying the altered control of Prl secretion in these patients, since plasma Prl stems from only normal lactotropes surrounding the tumoural tissue. The location of the tumour has a key role in influencing the pattern of Prl response to direct and indirect dynamic tests.