By the coupling of octylamine to the uronic acid function of morphine-3-glucuronide (M3G) a new glycoconjugate (morphine-3-octylglucuronamide, M3GOAM) was prepared. When assayed in both rats and mice up to ng/kg (ip) doses none of the animals survived. The aliphatic octyl chain may be the lethal factor since a closely related derivative (M3GNH2), was not toxic and showed similar opioid antagonist properties than naloxone. (C) 2004 Elsevier Ltd. All rights reserved.