BACKGROUND: Succinylcholine is the muscle relaxant of choice for rapid endotracheal intubation, but may produce many side effects such as hyperkalemia, myalgia, increase intraocular pressure. Nondepolarizing muscle relaxants were used instead of succinylcholine, still late onset time was be dangerous. For this reason, priming principle was reported and applied to rapid intubation using nondepolarizing muscle relaxation. We studied the effect of priming with vecuronium and atracurium on elderly and young patients. METHODS: We were randomly assigned 40 patients and observed the effects of priming doses of vecuronium (0.01 mg/kg) and atracurium (0.05 mg/kg). Ten young (20-35 yrs) and ten elderly (65-75 yrs) patients were each placed in vecuronium and atracurium group. Arterial blood gas analysis and train of four (TOF) were determined before priming. All tests were performed again 4 min after vecuronium and 3 min after atracurium. We asked for symptoms and signs of muscle weakness. RESULTS: In arterial blood gas analysis and TOF ratio were decreased in both groups. There is no significant difference between two groups in all tests. PaO2 and TOF ratio were reduced more in elderly patients, significantly (P<0.05). Symptoms and signs of muscle weakness such as ptosis, dizziness, diplopia, swallowing difficulty and respiratory difficulty in elderly patients were more frequent than in young patients. CONCLUSIONS: Priming doses of vecuronium and atracurium produced greater decrease in muscle strength, PaO2 and TOF ratio in the elderly than in their younger counterparts. So using priming method in elderly patients, we need adequate pre-oxygenation and thorough monitoring before endotracheal intubation.