ABSTRACT:The objective of this study was to investigate short-and long-term effects of early immunocastration with Improvac ® vaccine, administered in two doses, at ages eight and 14 weeks, on testicular histology in pigs slaughtered eight or 15 weeks after the second dose. We hypothesised that the effectiveness of early vaccination could be diminished by late application of the booster dose and/or delayed time of slaughter. Thirty non-castrated male pigs of a commercial hybrid breed were used in this study. Pigs (n = 15) in the control group (NOCA) remained intact throughout the study. Pigs (n = 15) in the experimental group (IMCA) were administered Improvac in two doses: a priming dose at eight weeks and a booster dose at 14 weeks. Subsequently, nine of the IMCA pigs were slaughtered at eight weeks and the remaining six at 15 weeks after the second dose. In NOCA pigs, we observed normal spermatogenesis in the tubuli seminiferi and many prominent interstitial endocrine (Leydig) cells. In IMCA 8 pigs, there was a noticeable decrease in the diameter and area of seminiferous tubules and spermatogenesis was absent. Interstitial endocrine cells appeared atrophied with pyknotic nuclei. In IMCA 15 pigs, we observed a larger diameter of tubuli, thickened germinal epithelium and larger and more numerous interstitial endocrine cells when compared to IMCA 8 . In conclusion, these results demonstrate that early immunocastration with Improvac disrupts spermatogenesis and reduces the number and size of interstitial endocrine cells. This indicates that vaccination at an age of eight weeks and again at 14 weeks in pigs causes disruption of testicular histology and spermatogenesis at least through the subsequent 15 weeks.