Physiological activity load is one of the main criteria for efficient training programming. The problem of the study is reflected in the fact that, although boxing and kickboxing belong to the group of polystructural sports in which the result is achieved by similar technical-tactical means, there are still certain differences in the physiological load of the competitive performance itself. The aim of this research was to compare the physiological load indicators between competitive performances in boxing and kickboxing. Extremely valuable results were obtained on a sample of 20 top athletes who participated in competitive performances in these two sports. Results of descriptive statistics and t-test, which we used to compare the obtained values of heart rate, serum lactate, creatine kinase and lactate dehydrogenase, proved that there are differences in all indicators of physiological and biochemical load between boxing and kickboxing, and we can conclude that in kickboxing there is still a slightly greater load on the body. Some of the differences were only numerically, and some are statistically significant. Regarding the heart rate, in both sports the load continuously increases during all three rounds, and the oscillations in the heart rate are more pronounced in kickboxing than in boxing. The indicator of larger oscillations in the heart rate is also the reason why during the kickboxing fight it was not possible to continuously maintain the progressive load due to the greater global organism acidity. Regarding the lactate and lactate dehydrogenase variables, there is also a statistically significant difference in favour of kickboxing, while in the creatine kinase variable, only a numerical difference was observed. During a kickboxing fight, foot kicking techniques and knee kicking techniques are used, which are not allowed in boxing, so more numerous muscle groups are used to mobilize these extremities, the activation of which then causes greater acidification of the body. Additional acidification in kickboxing is caused by a larger mass of the lower extremities, which, when colliding with the opponent's body, leaves greater consequences, which can then be seen in the obtained results of physiological and biochemical stress. Based on the general goal, partial goals of the research were defined for which two hypotheses were set. The first hypothesis that the physiological load in kickboxing is statistically significantly higher than in boxing was confirmed, but still only partially. Namely, a statistically significantly higher physiological load in kickboxing compared to boxing was confirmed in two of the four parameters (lactate and lactate dehydrogenase), while there was only a numerical difference in creatine kinase. In terms of heart rate, a numerical difference was recorded in favour of boxing, which reached statistical significance only in the third round. The second hypothesis, that there is a significant connection between the value of oxygen intake and the level of the anaerobic threshold with success in boxing and kickboxing, was fully confirmed by the analysis of spiroergometric indicators. This research represents a significant step forward, both due to the design of the study (direct comparison of physiological and metabolic changes during competitive boxing and kickboxing performances) and the number of subject’s involved, elite Croatian athletes. Based on the results, new knowledge was gained that will be useful in training processes in boxing and kickboxing.