Selected haematological, blood chemical and serological variables were investigated in healthy Thoroughbreds (n = 45) in training. Haemoglobin concentration, haematocrit, red, white and differential cell counts as well as serum concentrations of total and ionized calcium, sodium, potassium, chloride, urea, creatinine, total protein, albumin, inorganic phosphorus, total bilirubin, iron, glucose, magnesium, alkaline phosphatase, gamma-glutamyltransferase, lactate dehydrogenase, aspartate transaminase, alanine transaminase and creatine kinase were found to be within ranges previously reported for horses. No statistically significant difference was found between the haematocrit (Ht) of horses n = 44; mean = 0.44; SD = 0.02) of different performance or between those of different age groups. A significant difference was found between the Ht of males (mean = 0.43; SD = 0.02) and females (mean = 0.45; SD = 0.02) and between quiet (mean = 0.44; SD = 0.02) and excitable (mean = 0.46; SD = 0.02) horses. No significant difference in red cell potassium concentration was found between horses of different performance. Cortisol, insulin, parathormone (C-terminal), aldosterone and folate concentrations respectively varied between 89-204 (mean = 144.4; SD = 25.47) nmol l-1, 4.2-23 (mean = 10; SD = 4.30) m U l-1, 65.2-91.4 (mean = 79.46; SD = 9.34) pmol l-1, less than 138 to 379 pmol l-1 and 9.4-21.5 (mean 14.5; SD = 2.87) nmol l-1. Vit B12 concentrations exceeded 1400 pmol l-1. Blood lead concentrations in all animals were below 15 ug l-1. Fifteen (33.3%) of the horses were carriers of babesiosis. Laboratory findings concerning these horses did not differ from those of the other horses.