Plasma fluoride (F) concentration and urinary output of F were studied in 4 children, 4, 5, 12 and 14 years of age, after oral application of a F-containing varnish (Duraphat®). The F dose was found to be 2.3–3.0 mg for the younger children and 5.0–5.2 mg for the older ones. The highest plasma F concentration varied between 60 and 120 ng F/ml and was seen within 2 h of treatment. The urinary output of F during 12 h after varnish application was about 550 μg F/12 h for the younger children and 1,100 μg for the older ones. The plasma levels found are far below those considered