The effects of adrenaline and isoprenaline on K+ contractures of curarized tonic skeletal fibers were investigated. The K+ contractures of tonic fibers have a peak tension followed by a sustained tension. The peak tension and total tension (the tension-time integral--area--of K+ contractures) were increased by adrenaline and isoprenaline. The resting potential of tonic skeletal fibers were unaffected by adrenaline. The calcium channel blocker (cadmium and nifedipine) greatly blocked the effects of adrenaline on the peak and total tension of K+ contractures. On the other hand, the peak and sustained tensions of K+ contractures were greatly reduced in Ca(2+)-free solution, but, the peak tension recovered when the fibers were pre-incubated in adrenaline. It is proposed that adrenergic modulation of tension in tonic skeletal muscle fibers could be related with the modulation of Ca2+ channels and/or Ca2+ release from the sarcoplasmic reticulum.