The role and influence of football referees is very important; they are regarded as the 23rd player on the field. The aim of our research was to determine the nature and quantity of on-field activities of football referees in the first Slovenian division and in comparison with referees at the highest level and from other elite football countries (Italy and England). The main focus of the study was conducted on 12 first division referees (age 37±5.2), representing 75% of the population. We examined the heart rate data from the 1.SNL matches of the 2017/2018 season, collected through the Polar Flow online system. The results show, that the referees in-game work rate is between 70% and 90% of their HRmax. Findings show, that the heart rate of the Slovenian referees is statistically significantly lower in the second half of the game (p = 0,000) by 3,11 beats per minute. We also found that the Slovenian referees, with the exception of the second half of the games, have an average statistical significance in higher heart rate, compared to the results of the referees at the European Championship 2000 (p = 0,025), but lower heart rate compared to English and Italian first division referees (p = 0,000). Based on the results we can conclude that the Slovenian referees have good physical fitness levels; comparable to international referees at the highest level.