Terguride has been widely used for the treatment of hyperprolactinemia via partial agonistic action on dopamine D2 receptors in the pituitary. The present study analyzed retrospectively the dopamine D2 receptor binding occupancy (phi) of terguride. The average phi value was estimated to be 14.1% after oral administration of the average/standard therapeutic dose of terguride. Taking the intrinsic activity (alpha) into consideration, the value of alpha. phi was 2.33%. These results suggest that the antihyperprolactinemic effect of terguride was elicited despite the low receptor occupancy. Furthermore, we developed a pharmacokinetic/pharmacodynamic model for ascertaining the serum prolactin-lowering effect of terguride, considering both the reversible binding to D2 receptors and the effect on the increase rate in the prolactin level. The developed model fit well with the actual data. Although this model could be improved, it could explain the long duration of the antihyperprolactinemic activity of terguride and might be useful for designing its rational dosage regimen.