ABSTRACT
Blood samples were taken from eight Pantaneiro horses during a 76Km endurance ride. The horses were divided into two groups: 1- four horses kept on native pasture, without working and with no supplementation during one month before the ride, 2- four horses kept on native pasture with supplementation and submitted to work during one month before the ride. Serum concentration of total protein, albumin, sodium, potassium, chloride, calcium and phosphorus were measured. Samples were taken before the ride (preride), during the mid point (midride), at the end of the ride (postride) and after a 30-minute recovery period (rest). Sweat samples were collected from five horses at the end of the ride to measure sodium, potassium, and chloride. In the groups, there was a significant decrease in calcium and potassium, and an increase in sodium and phosphorus during the ride. Heart rate values after 30 minutes of rest indicated a good recovery response