Fluid and solute movements across rabbit distal colonic mucosa were investigated to determine the solute concentration of the absorbate and to characterize the absorption process. With isotonic mucosal and serosal bathing solutions the osmolality of the absorbate was 492 +/- 22 mosmol/kg. When the absorbate concentration was corrected for passive components of water and salt flux, the concentration of the "active" component was 1,062 +/- 206 mosmol/kg. Octanol (2 mM) caused a decrease in net solute and water absorption and in the concentration of the absorbate. Cytochalasin D (20 microM) increased the absorbate tonicity and decreased the hydraulic conductance of the active route for water movement. Correlations were made with measurements of the total crypt and crypt luminal cross-sectional areas. Ouabain, octanol, deoxycholate, and theophylline increase the cross-sectional area of the crypt lumen relative to that of the total crypt. These findings suggest that the hypertonic component of fluid absorption is via the crypts. The large osmotic pressure across the crypt wall could provide the considerable fluid tension (5 atm) required to dehydrate feces.