Meropenem, a carbapenem, has a broad spectrum of activity that includes Q -lactamaseproducing organisms, and are used frequently in treating various infections. Compared with imipenem/cilastatin, meropenem has a lower affinity for the r -butyric acid (GABA) receptor and thus a lower potential for inducing seizures. This makes meropenem, even at higher dosages and costs, suitable for the treatment of patients with infections of the central nervous system and for the treatment of infections in patients with high risk of seizure attack In 3 neurosurgical patients using valproic acid to control seizure, however, we experienced significant decrease of serum valproic acid concentration during concomitant use of meropenem. We report here our observations of decreases in the serum concentrations of valproic acid to sub-therapeutic levels in three adult neurosurgical patients during concomitant therapy with meropenem.