The variability of the post-exercise values (PEV) of blood lactate and of some other blood constituents were studied in 8 healthy and fit jumpers through 5 national competitions of equal difficulty. The effects of factors such as the individual, performance and competition on these PEV were analysed. Venous blood was sampled immediately after the 5 show-jumping contests and was analysed for packed cell volume (PCV), blood lactate, blood glucose (GLU), total plasma protein (TPP), bicarbonate (HCO3-), sodium (Na+), potassium (K+), chloride (Cl-), calcium (Ca2+), serum activities of lactate dehydrogenase (LDH), creatine phosphokinase (CPK), glutamic-oxalacetic transminase (GOT) and gamma glutamyl transferase (gamma GT). PEV of these 13 blood parameters were analysed using a fixed linear model which included the effect of horse, score and competition and their respective interactions. The analysis of the influence of each of the 3 fixed variables showed that (1) there was no relationship between the PEV variations and the score; (2) there were significant (P less than 0.05) variations induced by the competition in Na+, Ca2+, TPP, LDH and GLU, and (3) there were significant interindividual variations as regards Ca2+, TPP, CPK, gamma GT and PCV with P less than 0.05, LDH with P less than 0.01 and blood lactate with P less than 0.001.(ABSTRACT TRUNCATED AT 250 WORDS)