Sixteen ram lambs (5 mo old, average BW = 49 +/- 1.4 kg) received a control diet (50% concentrate, 13.1% CP) or a diet containing 7, 14, or 21% locoweed (LW, DM basis). Rams were housed in individual pens (2.5 x 3.5 m) in an enclosed facility (14 h light, 10 h dark) with free access to feed and water. After 35 d of LW feeding, rams fed the 21% LW diet ate 1.3 to 1.6 kg/d, whereas rams fed the other three diets ate 1.6 to 2.0 kg/d. On d 35, BW (mean +/- SE) were 58.3, 57.2, 57.2, and 55.4 (+/- 1.5 kg/d) for ram lambs receiving 0, 7, 14, and 21% LW, respectively (P>> .20). An inverse relationship for BW (P < .10) and gain (P = .04) was observed 1 mo after LW feeding ended relative to amount of LW in the diet. Serum alkaline phosphatase increased with increasing LW (P < .05), and thyroxine concentration was decreased (P < .05) by LW ingestion. On d 35, serum LH averaged 6.8, 9.9, 11.6, and 9.9 (+/- 1.8) ng/mL (P = .56), whereas testosterone averaged 3.5, 2.6, 3.0, and 1.6 (+/- .5) ng/mL (linear effect of LW, P = .05) over a 5-h period after GnRH injection (i.m., 50 microg) in lambs fed 0, 7, 14, and 21% locoweed, respectively. Semen volume, sperm motility, sperm cell concentration, and percentage of abnormal cells did not differ (P>> .50) on d 35. One month after the end of LW feeding, a linear (P = .06) decrease in sperm motility and scrotal circumference was observed relative to percentage of prior dietary LW. These data suggest that LW exerts adverse effects on the testes of young rams that may not be evident until several weeks after LW consumption ends.