The objective was to determine the effects of the opioid peptide receptor antagonist, naloxone on circulating concentrations of luteinizing hormone (LH) and testosterone in boars characterized as having high (n=8) or low libido (n=8) based on the willingness to mount an artificial sow and allow semen collection. On the day of the experiment, blood was sampled every 15 min for 4 h before and 4 h after i.v. injection of naloxone (1 mg/kg body weight). After naloxone treatment, a libido status by time interaction was detected and concentrations of LH within 15 min after treatment were greater (p<0.05) for High-libido boars than for Low-libido boars. Concentrations of testosterone were highly variable amongst boars and there were no effects of libido status (p=0.66) or libido status by time (p=0.66). There was, however, an effect of time (p<or=0.01), and concentrations of testosterone in samples collected between 0.5 and 1.25 h after naloxone were greater than concentrations in samples collected prior to injection. In summary, the responsiveness of the hypothalamic-gonadotropic-gonadal axis to opioid receptor antagonism was heightened in boars displaying a high level of sexual motivation.