Sperm motility, acrosome morphology, changes determined by the vital-lethal test and aspartate aminotransferase (AST) concentration in semen plasma were evaluated in the semen of four boars; the semen was stored for six years. No statistically significant changes in the percentage of motile spermatozoa were indicated when sperm motility was evaluated after four and six years of semen storage in liquid nitrogen. Neither did the fluctuation of the changes found on the basis of the vital-lethal test go beyond statistically insignificant values. After semen sample thawing in the BTS medium, the motility of spermatozoa was found to be somewhat higher than after thawing in the INRA-ITP medium, but after the termination of the thermoresistance test both media appeared to be equally effective. The AST level of the semen samples stored for four years was just slightly up on the initial values. After thawing in the BTS medium, AST level increased by 0.03 microcatal per litre of semen plasma, and in the INTRA-ITP medium by 0.06 microcatal per litre of semen plasma. The insemination of five sows with the semen stored for six years results in conception of two sows, i. e. 40%, and the average litter size was 7.5 piglets. It can be derived from the results that six years of boar semen storage in liquid nitrogen cause no further substantial changes in the structural and functional characteristics of spermatozoa.