Exposure of 100-d old rats to 1.3-dinitrobenzene (m-DNB) at dosages up to 48 mg/kg resulted in disruption of spermatogenesis as measured by flow cytometry (FCM) of acridine orange-stained sperm and testis cells. One day (d 1) after a single exposure to 48 mg/kg m-DNB. FCM measurements of caput epididymal fluid cells demonstrated the presence of testicular germinal epithelial cells apparently sloughed off into the epididymis. Also, at d 1 after the same exposure, a decrease in pachytene spermatocytes was observed. By d 16 after exposure to 32 or 48 mg/kg, testicular damage was evidenced by an alteration of cell type ratios in FCM-analyzed populations of testicular cells. Extensive recovery of cell type ratios occurred by d 32. At d 16, dosages of 32 and 48 mg/kg caused alterations of sperm chromatin structure as determined by the flow cytometric sperm chromatin structure assay (SCSA); 48 mg/kg caused alterations at both d 16 and d 32. Exposure to m-DNB caused a dose response increase in percent sperm head morphology abnormalities (%ABN) assessed in cauda epididymal and vas sperm. (Copyright (c) 1989 by Hemisphere Publishing Corporation.)