The distribution of lactate between blood cells and plasma, lactate concentration in whole blood and plasma, concentration of haemoglobin and packed cell volume (PCV) were measured in Standardbred and Finnhorse trotters. Red blood cell (RBC) lactate was calculated from whole blood and plasma lactate concentrations and PCV. Plasma lactate concentration correlated positively with whole blood lactate concentration over a range of exercise intensities (r = 0.972; P < 0.001). Similarly, plasma lactate concentration correlated with the calculated RBC lactate concentration both after the standardised submaximal exercise test (SET) and races, although there was individual variation in gradients. The slope of regression after the SET increased with sampling time, but such an effect was not found after the races. The mean corpuscular haemoglobin concentration did not show any correlation with RBC lactate concentration. Based on these results, we suggest that transport of lactate to erythrocytes occurs rapidly and is mainly determined by plasma lactate concentration. The great variation between individuals and individual similarity of plasma-cell concentration differences after submaximal and maximal exercise indicates that distribution of lactate between plasma and erythrocytes depends not only on plasma lactate concentration. Other factors, such as lactate transporter, may be important.