To determine the effect of low protein intake on boar libido, semen characteristics, and plasma hormone concentrations, 20 crossbred boars (1 yr of age) were divided into 10 littermate pairs, and boars from within pairs were fed 44 g/kg of BW.75 per day of either a low-protein diet (7% CP) or a control diet (16% CP) with the same energy content (3.41 Mcal of ME/kg). During the first 16 wk and from wk 19 to 23, semen was collected two times per week. During wk 17 and 18, boars were subdivided within dietary treatment and semen was collected either two or seven times per week. Blood samples were collected at 12-min intervals for 6 h before and 1 h after an intravenous injection of GnRH (375 ng/kg of BW) during wk 24. All plasma samples were analyzed for LH and pooled samples were analyzed for estradiol-17 beta and testosterone. Boars with low protein intakes required more time to start ejaculation (P = .11, wk 0 through 7; P < .04, after wk 7), had a shorter duration of ejaculation (P < .09, wk 19 through 23), and had reduced semen volumes (P < .01, after wk 7) compared with boars on the control treatment. There was no interaction between dietary treatment and semen collection frequency (P > .39) for any of the semen or libido measurements. Testosterone and LH concentrations were not affected by protein intake (P > .5). However, concentration of estradiol-17 beta was greater in boars fed the control diet than in boars fed the low-protein diet (582 vs 202 pg/mL, respectively; P < .08). Estrogen concentrations in boars were negatively correlated with the time required for the boar to start ejaculating (r2 = .72). Boars with low protein intakes had reduced libido and semen volume. This reduction in libido and semen volume may be a result of a decrease in estradiol-17 beta concentration in circulation.