The aim of this work was to evaluate the effects of the race training of young trotters on their heart rates (HR) and the levels of chosen blood parameters. 39 Standardbred horses 1.5 – 6 years old divided into three age groups were investigated. During the research, horses trotted on the distance of 15 km with the mean speed of 4.0 – 6.2 m·s^-1. Three jugular venous blood samples were taken for each horse: at rest, just after the end of training session and 30 minutes later. Hemoglobin (Hb) and lactic acid (LA) blood concentrations were determined as well as plasma level of glucose (Glc), glycerol, free fatty acids (FFA), uric acid (UA), total plasma proteins (tPP) and activity of lactate dehydrogenase (LDH). Speed and HR during the training session were recorded by a Polar speed and heart-rate telemeters. In the group of the youngest horses the lowest blood Hb concentration and the highest plasma Glc level were determined at rest. In this group, the highest increase of tPP and LDH was observed during the training sessions. Moreover, the values of Hb, tPP and LDH remained elevated after exercise. In groups of two-year-old and older trotters, the resting blood Hb level was higher than in the group of yearlings. Plasma glycerol and FFA values determined after exercise in three-year-old and older horses were lover than in groups of two-year-old and younger foals. The applied training process had a beneficial influence on the fitness of the investigated horses.

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