A possible interaction between betamethasone and vecuronium was examined in 20 rat phrenic nerve-hemidiaphragm preparations. Ten preparations were bathed in a physiological solution with betamethasone 1 mumol litre-1 added and, after a 30-min period were exposed to vecuronium at concentrations of 4, 6, 8 and 10 mumol litre-1 with vecuronium free washings between each exposure. Ten control experiments were performed also using a betamethasone-free bathing solution. In comparison with control, the betamethasone group had significantly (P = 0.0008) less depression of muscle contraction (twitch) force at all concentrations of vecuronium. The calculated ED50 (50% depression of muscle contraction force) was 5.65 mumol litre-1 for controls and 7.39 mumol litre-1 for betamethasone-pretreated preparations. This study confirms our previous clinical observations that an interaction occurs between vecuronium and betamethasone which is characterized by resistance to neuromuscular block.