Fluxes of K from mucosa to serosa or serosa to mucosa have been examined in stripped preparations of rabbit proximal and distal colon in vitro under short-circuit conditions in Ussing chambers. Results from these studies demonstrate that steady-state radioisotopic fluxes of K are achieved after 90 min and remain constant for at least 2 h. Determination of the K concentration dependence of the serosal-to-mucosal K flux revealed that this flux contains both saturable and nonsaturable components. Addition of ouabain (0.1 mM) abolished the saturable component of the serosal-to-mucosal K flux. The mucosal-to-serosal K flux is a linear function of K concentration between 1 and 20 mM under basal conditions. In paired tissues, serosal-to-mucosal K flux is always greater than mucosal-to-serosal flux under basal conditions resulting in net K secretion. However, addition of barium (2 mM) to the mucosal or serosal bathing solution had no significant effect on either unidirectional or net K fluxes. In addition, mucosal bumetanide (0.1 mM) or removal of Cl from both bathing solutions had no significant effect on unidirectional or net K fluxes. In rabbit distal colon, Cl removal from the bathing solutions significantly reduced serosal-to-mucosal K flux, resulting in net K absorption. These results indicate that rabbit proximal colon like rabbit distal colon actively secretes K. However, unlike distal colon the proximal colon does not possess an active K uptake mechanism at the apical cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)