Objectives:
Endurance means to be insensitive to tiredness caused by physical effort. This is the ability to put a great effort and to continue it for a long time (above 60 seconds). The outcome of efficiency level is similar among girls and boys untill puberty. The highest rate of growth in endurance among boys is between the ages 6–10 years; however, girls aged 10 have greater endurance than the boys, while boys aged 12 and more are physically stronger. Objective. The aim of the study is evaluation of endurance in girls and boys aged 9–10 years attending the Ferdynand Marecki School in Supraśl.

Methodology:
The study included children who completed grade 3 Primary School from the Sports School Complex in Supraśl and nearby Primary School in Ogrodniczki. During the period 2003–2013, a total number of 470 children aged 9–10 were examined. Their endurance was evaluated using the race test at the distance of 500 meters, in accordance with the guidelines by the School Sports Association (1981).

Results:
The boys achieved slightly better results in running at the distance of 500 meters than the girls. Only in 2012 the girls proved to be better runners than their male colleagues. Both the girls and boys attending the cross-country skiing class achieved much better results at the distance of 500 meters than their contemporaries who continued education in a class with a profile other than sports. The shortest mean time of the race among both girls and boys was noted in 2003. However, the longest time for covering this distance was in 2011.

Conclusions:
The study confirmed higher endurance to physical effort among boys than girls. Children attending the crosscountry skiing class proved to be physically stronger than the students attending classes of other profiles.