To determine the effect of energy and protein intakes on boar libido, semen characteristics, and plasma hormone concentrations, 24 crossbred boars (1 yr of age) were allotted to one of three dietary treatments. Two protein levels (7.7 and 18.1 g/d of lysine) and two energy levels (6.1 and 7.7 Mcal/d of ME) were combined to create 1) low-energy and low-protein (ep), 2) low-energy and high-protein (eP), and 3) high-energy and high-protein (EP) treatments. Semen was collected two times per week for 27 wk. During wk 28, blood samples were collected before and after a intravenous injection of GnRH (375 ng/kg of BW). The EP boars had higher (P < .01) ADG than the eP boars (373 vs 169 g), which had higher (P < .01) ADG than the ep boars (169 vs 92 g). Fewer EP and eP boars (zero of eight and two of eight, respectively) than ep boars (five of eight) consistently refused to mount the collection dummy (P < .05). During the final 20 wk of the semen-collection period, EP and eP boars had 33% longer durations of ejaculation (P < .04), had 60% greater semen and gelatinous fraction volumes (P < .03), but had 25% lower sperm concentration in their semen (P < or = .12) than ep boars. The EP boars produced 38% more sperm than the ep boars (P < .10). Baseline LH concentration, LH pulse frequency, LH pulse amplitude, testosterone concentration, and estradiol-17 beta concentrations were not affected by treatment (P > .10).(ABSTRACT TRUNCATED AT 250 WORDS)