Present study was performed to investigate whether TRH or metoclopramide (MCP) loading test was useful for the diagnosis of so-called occulted or latent hyperprolactinemia (transient increase of serum prolactin levels more than 30 ng/ml during night; OHP). The circadian profiles of serum prolactin levels were examined in 31 women (age: 23-32 years old) whose BBT charts showed biphasic patterns. Blood samplings had been done every two hours through an intravenous indwelling catheter without any disturbances. And seven cases of the OHP were selected. Five cases of the control were also selected at random. Then, LH-RH (100 micrograms) and TRH (500 micrograms) loading test and LH-RH and MCP (10 mg) loading test were performed to these cases in the mid-luteal phase of the same menstrual cycle at interval of two or three days, and serum FSH, LH and prolactin levels (at 0, 30, 60, 90, 120 min. after the loading test) were determined by radioimmunoassay. Serum prolactin levels in the OHP group showed significant higher levels than those of the control from 22 to 6 o'clock (p less than 0.05-0.005). By the administration of 500 micrograms of TRH, serum prolactin levels of the OHP group increased significantly compared to those of the control at all sampling points (p less than 0.05-0.005), and also by the administration of 10 mg of MCP, the same result was obtained (p less than 0.05-0.02). The maximum peak of serum prolactin levels appeared at 30 min. after TRH or MCP loading.(ABSTRACT TRUNCATED AT 250 WORDS)