In order to estimate the mechanism of prolactin secretion using the kinetic model, serum prolactin concentrations in normal young men were measured by an immunoradiometric assay before and after intravenous administration of 500 micrograms of thyrotropin-releasing hormone (TRH). The kinetic model was applied to evaluate the secretion rate both before and after the administration of TRH. A good correlation between the calculated values and measured values of serum concentration was observed (r = 0.995); and the adequacy of the model was confirmed. A few minutes (1.97 +/- 0.57 min) after the administration of TRH, the secretion of prolactin was stimulated, and serum prolactin concentration was maximum at 12.7 +/- 2.1 min. The total amounts of secreted prolactin was 75.4 +/- 18.4 ng/ml, which corresponded to the amount that was secreted in 4.3 hours at the normal rate. The half-life of prolactin was 40.8 +/- 13.8 min, which is in good agreement with the previously reported values. We have already reported on the kinetic study on the TSH secretion after TRH. Comparing the concentration change of prolactin and TSH, which were reported previously, it was found that the stimulated secretion started almost simultaneously, but lasted longer with respect to TSH.