Five groups of horses were fed different diets of known trace mineral concentration for a minimum of six months. Copper (Cu), zinc (Zn) and manganese (Mn) concentrations were measured in livers of 125 yearling horses and kidneys of 81 yearling horses as an assessment of trace mineral status. Plasma Cu and Zn determinations were made for all horses.Mean hepatic Cu concentrations of horses fed diets containing 6.9 to 15.2 mg Cu/kg dry matter (DM) feed were 0.27 to 0.33 mumol/g DM tissue. Plasma Cu concentrations ranged between 22.8 to 28.3 mumol/L. There was no simple mathematical relationship between plasma and hepatic Cu concentrations. Mean hepatic Zn concentrations in horses fed diets containing 25.6 to 52.2 mg Zn/kg DM feed were determined to be between 2.75 to 2.91 mumol/g DM tissue. Mean plasma Zn concentrations in groups of horses were between 11.7 to 13.5 mumol/L. Plasma Zn concentrations were not indicative of hepatic Zn concentration. Hepatic Mn concentrations ranged between 0.13 and 0.14 mumol/g DM tissue.Renal Zn concentrations ranged between 1.55 to 1.63 mumol/g DM tissue and did not differ with diet. Mean renal Mn concentrations were 0.09 mumol/g DM tissue for all groups of horses. Renal Cu concentrations ranged from 0.36 to 0.47 mumol/g DM tissue and differed with diet.