With this study we evaluated the effects of the herb rosemary (Rosmarinus officinalis L. Lamiaceae) extract on the labeling of blood constituents with technetium-99m (Tc-99m) labeled sulphur colloid and on the biodistribution of Tc-99m-Sulphur Colloid in Wistar albino rats. For this purpose, two groups of animals (male wistar rats, 130140 g) were treated (1 mL) with a rosemary extract (750 mg/kg body wt., n=9) and water (control, n=9) separately by gavage for five days. Tc-99m-Sulphur Colloid was administrated by intravenous injection; organs/tissues were withdrawn and weighted. Blood was centrifuged, plasma and blood cells were isolated. The radioactivity was counted to calculate the percentage of activity per gram for each organ/tissue and percentage of activity in blood cells and plasma. A significant increase (p<0.05) in the uptake of Tc-99m-Sulphur Colloid in the liver after the treatment with rosemary extract was observed. These results indicate that the substances or metabolites of the rosemary extract would change the biodistribution of Tc-99m-Sulphur Colloid.