Seminal zinc was measured in normospermic and infertile patients by a new colorimetric method. Analysis of semen from 5 patients with bilateral agenesis of the vas deferens showed high levels of zinc (mean value 1411.2 micrograms/ml). Lower levels were found in 6 patients with monolateral congenital or acquired obstruction (695.2 micrograms/ml). The seminal zinc level in 6 patients who had had a vasectomy or who had an epididymal blockage (125.7 micrograms/ml) was approximately the same as in 41 controls (134.6 +/- 42 +/- SD). Zinc levels were reduced in hypogonadal patients. These data suggest that zinc is secreted mainly by the prostate, while vesicular, epididymal and testicular secretions are devoid of zinc. Evaluation of this metal therefore permits the diagnosis of patency of the seminal pathways, but does not permit identification of prostatitis. No correlation was found between sperm count or motility and the seminal zinc level, nor between the latter and varicocele.