In order to study the control and secretory capacity of the prostate gland and seminal vesicles, semen samples were collected at regular intervals. Despite a marked reduction in volume of samples collected at 24 hr intervals or shorter, the concentration of fructose (from seminal vesicles), transaminase, acid phosphatase, cholesterol (from prostate gland) and neuraminic acid (from both systems) was almost unchanged. This indicates that during emission of semen not only the composition of fluid from each system but also the relative amounts from each of them is kept constant.