Resistance to atracurium-induced neuromuscular blockade in patients with intractable seizure disorders treated with anticonvulsants. Previous studies have demonstrated that, with the exception of atracurium, resistance to the neuromuscular blocking effects of various muscle relaxants develops in patients receiving anticonvulsant therapy. We studied the effects of 0.5 mg/kg IV atracurium in 53 neurosurgical patients: 21 nonepileptic patients receiving no anticonvulsant therapy (MED = 0); 14 epileptic patients treated with carbamazepine for years (MED = 1); and 18 epileptic patients treated with carbamazepine plus either phenytoin or valproic acid for years (MED = 2). The evoked compound electromyogram of the adductor pollicis brevis was recorded, and results were analyzed using analysis of covariance, with weight and age as covariables. The onset time was not significantly different among the three groups. Times for recovery of baseline and train-of-four responses to stimuli were significantly shorter in the MED = 1 and MED = 2 groups than in control patients (MED = 0). The recovery index (time between 25% and 75% recovery of baseline electromyogram values) was progressively shorter in the three groups (MED = 0: 8.02 min; MED = 1: 5.93 min; MED = 2: 1.96 min; P less than 0.001). This study demonstrates that atracurium, when used on epileptic patients requiring long-term (that is, years of) anticonvulsant therapy, has a shorter duration of action than when used in nonepileptic patients. 