The effects of season and husbandry on the selenium (Se) status of four flocks, located in areas of Dyfed in Wales, were monitored over a 16-month period. The Se status of each flock was assessed from the activity of the Se-dependent enzyme glutathione peroxidase (EC 1. 11. 1. 9, GSH-Px), determined at 30°C in individual blood samples. The survey was initiated in January 1981 and samples were collected at monthly intervals from 18 ewes per flock.