We measured neutral endopeptidase (NEP) activity in serum from non-smoking healthy Japanese and in cerebrospinal fluid (CSF) from patients without neurological or inflammatory diseases. The serum NEP activity (sNEP) of 25 males and 25 females, aged 20 to 65 years, ranged from 0.003 to 1.62 pmole/min/microliter. There was no significant difference in sNEP activity between the sexes (male: 0.40 +/- 0.34 pmol/min/microliter vs female: 0.37 +/- 0.30, mean +/- S.D.). There was a significant positive correlation (p < 0.05) between sNEP and age. The NEP activity in the CSF (cNEP) ranged from 0.07 to 0.63 pmole/min/microliter. Male patients with benign prostate hypertrophy (BPH) showed cNEP activity of 0.21 +/- 0.11 pmol/min/microliter (n = 13), and female patients with myoma uteri (MU) or dysplasia of the uterus mucosa (DUM) showed activity of 0.32 +/- 0.20 (n = 5). There was no significant difference in cNEP activity between the sexes. Three patients with severe body pain showed cNEP activity of 0.21, 0.15, and 0.16 pmole/min/microliter, and these values were dissimilar from those of the BPH, MU, or DUM patients.