Three beta-adrenoceptor mediated effects were measured in vitro on tissues from guinea-pig: (a) relaxation of the trachea (mainly beta 2), (b) increase in the force of contraction of the papillary muscle (beta 1), and (c) depression of the subtetanic contractions of the soleus muscle (beta 2). The relaxant effect of ephedrine on the trachea was weak but was resistant to reserpine pretreatment. There was no appreciable agonistic effect of ephedrine on the soleus muscle. Ephedrine per se had a marked positive inotropic effect on the papillary muscle with a pD2 of about 6.0. This effect disappeared completely after pretreatment with reserpine. Ephedrine inhibited the response to isoprenaline. The apparent pA2 was about 4.8 in all tissues studied.