p-tert-Octylphenol (OP) is a degradation product of alkylphenol ethoxylates. OP is an endocrine disruptor known to bind to the estrogen receptor; however, effects on males are controversial. The objective of this study was to evaluate the effects of chronic exposure to OP on male reproduction. Adult Sprague-Dawley rats were administered OP for 60 d, representing 1.5 cycles of spermatogenesis. Experimental groups included a vehicle control, and three doses of OP (25, 50, or 125 mg/kg body weight [bw]) administered daily by gavage. There was a significant decrease in body weight in the 125-mg/kg group after 60 d of treatment. Both testicular and epididymal weights and histology were not altered by treatment with OP at any of the doses administered. There were no marked differences in cauda epididymal sperm counts at any doses; however, total percent sperm motility was significantly lower in rats exposed to the intermediate dose (50 mg/kg bw). There was an increase in percent static sperm cells in all OP-treated groups, with the intermediate dose (50 mg/kg) displaying a significantly higher proportion of static cells relative to untreated controls. Caput epididymal sperm motility was unaltered by OP treatment. Gene expression profiles of testes from control and high-dose-exposed rats indicate that 14 genes were modulated by at least twofold, although these changes were not statistically significant. Taken together, results from this study indicate that OP treatment of adult rats does not appear to exert major effects on male reproductive endpoints at relevant environmental exposure doses.