Adult male rats were dosed with 0 or 2000mg/kg of boric acid and killed at 2, 14, 28, or 57 d posttreatment, or dosed with 0, 250, 500, 1000, or 2000mg/kg of boric acid and killed at 14 d posttreatment. At d 14, structures which appeared to be enlarged atypical cytoplasmic lobes were observed in Stage VIII seminiferous tubules of rats dosed with 1000 and 2000mg/kg. Abnormal retention of Step 19 spermatids and residual bodies was also observed in Stage IX-XIII tubules of these rats. Abnormal caput epididymal sperm morphology, and reduced caput epididymal sperm reserves were observed at 1000mg/kg and higher. At d 28, rats dosed with 2000mg/kg exhibited continued retention of Step 19 spermatids into Stage X, abnormal caput and cauda sperm morphology and decreased percentages of motile cauda spermatozoa with reduced straight line swimming velocities. By d 57 retention of Step 19 spermatids into Stage X tubules was still present in some rats but the sperm parameters had returned to control values. The study shows that acute exposure to boric acid can adversely affect spermiation and sperm quality. These effects appeared readily reversible at the dosage levels tested.