This study was undertaken to assess differences between young and elderly patients with respect to the duration of antagonism of metocurine neuromuscular blockade by neostigmine (NEO) or pyridostigmine (PYR). Patients were given either NEO (seven elderly and ten young) or PYR (seven elderly and eight young) and received nitrous oxide in oxygen (60:40) and 1 MAC halothane. Neuromuscular transmission was assessed by using evoked compound electromyography. Metocurine 0.1 mg/kg was given, followed by a continuous infusion to achieve 90% reduction in baseline single twitch height. After at least 30 minutes, either NEO (0.07 mg/kg) or PYR (0.14 mg/kg) and atropine (0.02 mg/kg) were given IV. After injection of NEO or PYR, the duration of maximal response (DOMR) was recorded. Mean (+/- SE) ages were 38 +/- 5 and 68 +/- 2 years in the two groups of patients receiving PYR. In the elderly, PYR significantly prolonged DOMR compared to younger patients (35.3 +/- 8.2 vs 14.4 +/- 4.2 minutes, respectively). The mean ages in the two groups of patients receiving NEO were 41.5 +/- 4 and 72 +/- 2 years. The elderly group demonstrated a significant increase in the DOMR compared to younger patients (32 +/- 10 vs 11 +/- 2 minutes, respectively). It is concluded that, compared to younger patients, the duration of action of NEO and PYR in the aged patient is prolonged.