The effects of quercetin, flavone, catechin and chrysin on the naloxone-precipitated withdrawal contracture of the acute morphine-dependent guinea-pig ileum have been investigated in-vitro. After 4 min in-vitro exposure to morphine a strong contracture of guinea-pig isolated ileum was observed after the addition of naloxone. All the flavonoids, injected 10 min before morphine at concentrations between 10(-7) and 10(-5) M, were capable of blocking naloxone-induced contracture after exposure to morphine in a concentration-dependent fashion. IC50 values calculated for quercetin, flavone, catechin and chrysin were 2.7 x 10(-6), 7.3 x 10(-7), 8.5 x 10(-7) and 5.3 x 10(-6) M, respectively. These results suggest that flavonoids might play an important role in the control of morphine withdrawal.