Milacemide, an acylated prodrug of glycine, was able to increase the efficacy with which [+]-5-methyl-10,11-dihydro-5h-dibenzo[a,d]cyclohepten-5,10-imine meleate (MK 801) antagonized the electrical precipitation of seizures in mice. The mechanism of milacemide's potentiation of MK 801's antiseizure efficacy in intact mice is unclear; however, a glycine agonist selective for the strychnine-insensitive site on the NMDA receptor complex was also able to potentiate MK 801. The exciting possibility exists that an exogenous glycinergic intervention can potentiate NMDA-mediated neural transmission in intact animals.