“In vitro”epinephrine (10−6to 10−4M) stimulates the rat inferior rectus, one of the extraocular muscles (EOMs). When applied to resting muscles, it induces a slow rising tension in about one half of the preparations. The amplitude of this tension is equivalent to about 9% of the maximal tetanic response. This effect is blocked by propranolol (10−7to 10−6M). When epinephrine is applied prior to a depolarization of the muscle fibers with 30 mM K, the drug consistently induces a significant potentiation of the long lasting tonic tension normally evoked by that elevated K concentration. One possible explanation for these results is that epinephrine induces a slow rising tension by increasing a resting Ca conductance of the sarcolemma of the tonic or Felderstruktur fibers of EOM.