8,9-Dioxo-6-phenyl-1-aza-7-oxabicyclo[4.2.1]nonane (1) and 9,10-dioxo-7-phenyl-1-aza-8-oxabicyclo[5.2.1]decane (2), examples of anti-Bredt bicyclic 2,4-oxazolidinediones, were investigated as anticonvulsants in mice. Compound 2 was the more potent (anti-MES ED50 = 66 mg/kg), and its in vivo anti-MES effect was consistent with its in vitro potency of binding to the voltage-sensitive sodium channel (IC50 = 160 μM for the inhibition of binding of [3H]BTX-B), suggesting that 2 may be a new class I anticonvulsant. Several partial structures of 2, either monocyclic lactams or monocyclic 2,4-oxazolidinediones, were also evaluated in these assays, but no correlation was observed between sodium channel binding and anti-MES effects. A significant finding was that monocyclic 5-alkyl-5-phenyl-2,4- oxazolidinediones provided relatively potent, nontoxic, broad-spectrum anticonvulsants. © 1988, American Chemical Society. All rights reserved.