Isolated rectal gland tubules (n approximately equal to 1000) of dogfish (Squalus Acanthias) were perfused in vitro. Individual channels in the apical and basolateral cell membrane were recorded with the patch clamp method. K+-channels were present in excised membrane patches of the basolateral membrane in stimulated (dbcAMP + forskolin + adenosine) and in nonstimulated state. Cl- -channels were found only in patches of the apical cell membrane when the tubule was stimulated. Cell attached recordings and simultaneous transepithelial PD measurements were obtained while the segment was stimulated. It is shown that concomitant with the increase in lumen negative PD "silent" membrane patches of the apical cell membrane suddenly develop Cl- -channel activity. It is concluded that stimulation of rectal gland tubules "activates" Cl- -channels in the apical cell membrane.