As part of a study of the metabolic effects of long distance riding the results of biochemical analyses of blood samples taken from horses before, immediately after and one hour after an 80 km ride are reported. The results show that the horses were moderately dehydrated, they were working aerobically using fats as metabolic substrates and blood glucose was reduced. There was no evidence of post exercise ketosis and circulating alanine levels fell. Metabolic hormone levels are reported and are related to the availability of substrates for gluconeogenesis. There was evidence of reduced kidney and liver function which was showing little sign of recovery in the first hour after the ride.