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Extra-epididymal spermatozoa account for approximately a third of all spermatozoa found in the normal human ejaculate. Whilst remaining outside of the testes at core body temperature, the functional competence of spermatozoa, including cell motility and fertilizing capacity, diminishes. By examining spermatozoa found in the seminal fluid of recently vasectomized men, this study has investigated the nuclear changes that occur in spermatozoa whilst persisting in sites distal to the epididymis. Spectral recordings of spermatozoa stained with the nucleic acid dye, toluidine blue and the sperm chromatin structure assay (SCSA) were performed. Toluidine blue staining of human sperm DNA is an effective predictor of abnormal protamine disulphide crosslinking and chromatin condensation. Using flow cytometry, the SCSA determines the sensitivity of sperm DNA to acid-induced denaturation, providing a measure of chromatin and DNA damage. Abnormal protamine disulphide crosslinking and chromatin condensation was significantly higher in spermatozoa from patients after vasectomy when compared to normozoospermic controls (p < 0.01). Additionally, spermatozoa from vasectomized donors were significantly more sensitive to acid-induced denaturation than were normozoospermic donors (p < 0.05). The results indicate that spermatozoa surviving in extra-epididymal sites are more likely to possess DNA and chromatin abnormalities than those present in the testes and epididymis. These changes may partly explain the depletion of cell viability and fertilizing capacity of extra-epididymal spermatozoa which has been reported previously. [ABSTRACT FROM AUTHOR]