Compared to adults, children have longer simple reaction time (SRT) and choice reaction time (CRT) to simple and choice visual stimuli. Based on the available data, it can be concluded that this occurrence could be due to correlation between age and the information processing speed. Other factors may have an impact, too, such as the shorter attention span which children may have. Some studies indicate that karate favors the development of cognitive functions, including the RT. Therefore, the aim of the current study was to determine whether there is a difference in simple and choice reaction time between boys and girls of preadolescent and early adolescent age on the basic and premastery level of karate training. Testing involved 78 participants divided into four subgroups, in accordance with the gender and the level of training, using a custom designed RT measuring software, developed in LabVIEW 2012 software package. A two-way analysis of variance (Two-way ANOVA) confirmed no differences between boys' and girls' SRT scores, while the significant differences were found at the mastery criterion subsample level. When it comes to CRT, the differences were found at the boys' subsample level, as well as between genders on the basic level of training. It cannot be stated with absolute certainty whether the acquired differences are the result of karate training, some other physiological and cognitive phenomena or the maturation process. Consequently, the further research should involve a broader sample - the population of the physically inactive, as well as the children involved in other sports.