The effect of factors including the horses’ farm environment, their sex and age and whether they suffered from summer seasonal recurrent dermatitis (sweet itch) on the concentrations of zinc in the plasma, whole blood and blood cells of 104 Icelandic horses was investigated. Its concentration in plasma varied significantly between farms (P<0·01), but its concentration in blood and blood cells was not influenced by any of the variables. The concentration of zinc in the blood cells was 10·5 times greater than in plasma, but its concentration in plasma was not correlated with its concentration in whole blood or blood cells owing to the variability in the proportion of whole blood zinc present in plasma (relative plasma zinc), which ranged between 9 and 24 per cent. This variability was significantly influenced by a three-way interaction between farm, sex and sweet itch (P<0·05). Relative plasma zinc was positively correlated with absolute plasma zinc (r=0·78, P<0·001) and negatively correlated with whole blood and blood cellular zinc (r=–0·58, r=–0·71, P<0·001).