The concentration of prostate specific acid phosphatase (PAP) was significantly higher in serum specimens from prostatic venous plexus blood than from peripheral venous blood in 6 patients operated upon because of benign prostatic hypertrophy. This suggests that normally circulating PAP is secreted via the prostatic venous plexus. We also investigated the disappearance of PAP from the circulation after total prostatectomy and staging pelvic lymphadenectomies in 5 patients with nonmetastazing prostatic cancer and in 1 patient with bladder cancer. During the postmaximum period, serum PAP concentrations declined, following 2-exponential curves, the 1st mean half life of elimination being 1.2 hours (range 0.5--2.5 hours). It is possible that PAP released from the prostate during the operation was eliminated during the 1st period. The 2nd half life was found to be remarkably long (mean = 281 hours), and it may represent PAP bound by serum protein(s).