A slow-release selenium (Se) fertilizer was tested under grazing conditions with merino sheep, in a mediterranean environment on a lateritic podzolic and a non-calcic brown soil in Western Australia. The experimental site was located 80 km east of Perth, Western Australia. Some sheep were maintained on the pasture for 4 yr and other low selenium sheep were introduced at various times over 5 yr to distinguish carryover effects within the sheep from those in the pasture/soil. A single application of the slow-release Se fertilizer based on BaSeO4 at 10 g Se ha-1, maintained adequate Se concentration in the blood to prevent subclinical Se deficiency for 4 yr. A single application of Na2SeO4 fertilizer applied at the same rate was effective for only 15 mo. The difference in solubility between the quick (Na2SeO4) and the slow-release form (BaSeO4) of Se allows alternative management strategies. The quick-release form could be used annually in smaller quantities (2-3 g Se ha-1) where pastures are rotated with crops that do not require Se. The slow-release form at higher application rates (10 g Se ha-1) is more suited to long-term pasture areas which would therefore require infrequent applications of Se. Slow-release Se fertilizers that are controlled by dissolution are well adapted to the mediterranean environment where crop growth is restricted to the cool wet winter months and there is very little soil moisture or plant growth during the summer.