Os teopenia , r ickets , and/or o s t eoma lac i a have been observed in patients chronical ly t reated with ant iconvulsant drugs. Part of these bone disorders have been related to an ant iconvulsantinduced al terat ion of vi tamin D metabol ism [1, 2]. H o w e v e r , o ther more direct effects of long-term barbiturate therapy on bone cannot be excluded [3]. In vitro, 10-3-10 4 M diphenylhydanto in and phenobarbi ta l have been shown to inhibit the calcium release from fetal long bones [4]. L o w e r concentra t ions have not been investigated. In order to test a possible local influence of barbiturates on bone resorpt ion, we studied the effects of low-dosage sodium 5,5 diethylbarbi turate (Barbital Na) on the formation of resorpt ion pits by freshly isolated rabbit osteoclasts and in rabbit bone mar row cultures.