The objective of the experiment was to determine the effect of natural substances as a potential substitute for antibiotics in the boar extender on the semen survival time. Thirteen natural substances were tested. Natural substances were dissolved in 4% DMSO with a minimum bactericidal effective concentration of between 300 and 4,800 µg/ml. The control sample was diluted with a semen dilution ratio of 1:1 in a BTS extender without antibiotics. Sperm motility was evaluated at 0h, 1h and 24h after semen dilution. The best sperm motility was only recorded at thymol 25 % and carvacrol 40 % after 24h storage time. Other tested natural substances had sperm motility between 0 and 5 %. The sperm motility was significantly decreased over the storage time for all the tested natural substances (P < 0.01). Results of the present study indicate a negative effect of the concentrations of tested natural substances on boar semen survival time. At present, practical utilization of the natural substances we tested as a potential substitute for antibiotics in boar semen extenders is not possible owing to its reduced sperm motility.