This study was undertaken to determine whether a methanol extract of endophyte-infected tall fescue seed contained the factor(s) that were responsible for the decrease in male reproductive potential when rats were fed toxic tall fescue seed. Twenty-four Sprague-Dawley male rats (ca. 225 g.) were randomly allocated to one of three dietary treatments containing 50% (w/w) rodent chow and 50%: (1) noninfected tall fescue seed; (2) noninfected tall fescue seed treated with concentrated methanol effluent from serial methanol extraction of infected seed; and (3) the extracted seed residue of seed that was greater than 95% infected by Acremonium coenophialum. Parameters assessed were testicular weight, epidydimal weight, parenchymal weight, progressive sperm motility, sperm motility, sperm count and daily sperm production potential. Significant reduction (P less than 0.05) for feed intake, average daily gain, testicular weight, epidydimal weight and daily sperm production potential were observed for treatment 2. These data suggest that the serial methanol extract contains the factor(s) responsible for the lowered reproductive potential in male rats. The extract contained 15,578 micrograms/ml of pyrrolizidine alkaloids and only trace amounts of ergopeptides. These results suggest that N-formyl and N-acetyl loline may play an important role in the fescue toxicity noted in the exposed rats.