Large White boars (n = 120) were assigned to hemi-orchidectomy at 60, 100, 140 or 300 days of age or as intact controls throughout the 450 day observation period to evaluate the effects of unilateral testicular hypertrophy on libido (reaction time – RT), ejaculation rate (EJACS) and sperm quality in hemi-orchidectomised males. Hemi-orchidectomy of the experimental males involved the surgical removal of the left testis. Sperm viability tests were performed from aliquots of fresh semen following libido and sexual capacity tests. Hemi-orchidectomy of boars at days 60, 100, 140 and 300 of age caused the weight of the remaining testis to hypertrophise by 15.1%, 124.4%, 346% and 483.2%, respectively, as compared to the uncastrated control at 450 days. All the measurements were taken at the end of the study period (day 450). Total daily sperm production per testis (TDSP) increased to 154%, 160.8, 184.3% and 247%, respectively, with this increase being dramatic (247%) in the 300 day old boars. At day 450 compensatory testicular hypertrophy in boars hemi-orchidectomised at day 300 resulted in the highest number of mounts (15.6 ± 0.8 per 30 min), ejaculation rates (9.7 ± 0.9 per 30 min), total sperm count (94.1 ± 4.7 x 109), sperm motility (90.3 ± 1.7%) and normal acrosome morphology (92.2 ± 2.3%), with the shortest RT (2.2 ± 0.5 min). Mounting frequency, RT, EJACS, total sperm, sperm motility and normal acrosome morphology were correlated with testis weight, TDSP, seminiferous tubular diameter, tubular length, tubular mass and total Leydig cell mass. Results suggest that hemi-orchidectomy induces compensatory testicular hypertrophy, leading to significant improvement in the reproductive output of boars. These responses became more pronounced the older the boars were when orchidectomised.