In male and female rats, R-goitrin treatment increased the relative liver weights; in some of the enlarged livers a high incidence of mitotic figures was observed. R-goitrin treatment increased the relative thyroid weight, more in male rats than in females. The thyroids of R-goitrin treated rats were either hyperplastic or had most follicles filled with pale-staining colloid and rarely follicles with normal, well-stained colloid. At certain doses, R-goitrin increased serum triglycerides, cholesterol, total protein, albumin and calcium, but it decreased serum thyroxine and urea. However, most of these changes in serum chemistry were small. R-goitrin caused a temporary increase in urinary ascorbic acid output in both sexes, but the liver ascorbic acid level was increased only in female rats. The duration of pentobarbital-induced sleep was significantly prolonged by R-goitrin pretreatment only in male rats.