Fluids were collected from the proximal vas deferens of 18 normal fertile men and 32 vasectomized men during vasectomy or vasovasostomy, respectively, and the protein concentration and pattern of proteins were then analysed by sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE). In normal fertile men and vasectomized men there were no significant differences in total protein concentrations between fluids from the left and right vas deferens. The total protein concentration of proximal vas fluid from vasectomized men (82.8 +/- 7.5 mg/ml; mean +/- SEM) was significantly higher than in normal fertile men (36.8 +/- 4.5 mg/ml). In vasectomized men there was no significant correlation between the total protein concentration in vas fluid and the duration of vasectomy. The patterns of protein bands in proximal vas fluid differed from those in seminal plasma. There was no relationship between the pattern of protein bands in vas fluid and the duration of vasectomy. Five major groups of proteins from proximal vas fluid were identified and no new major proteins were found in proximal vas fluid were identified and no new major proteins were found in proximal vas fluid after vasectomy. The percentage of 80K and 67K protein bands in vasectomized men was higher than that in normal fertile men. It is suggested that, after vasectomy, the physiological environment of the proximal vas deferens may be changed. One of these changes may be the higher total protein concentration (especially the 80K and 67K proteins) in vas fluid.