Increased renal ammoniagenesis is thought to be a causative factor for renal hypertrophy which occurs in several disorders accompanied by metabolic acidosis. We studied the influence of ammonia on the polarized release of degradation products of endogenous proteins in LLC-PK<sub>1</sub> cells. Release of acid-soluble radioactivity to the extracellular space decreased under the influence of NH<sub>4</sub>Cl with a remarkable transient reduction of basolateral release. Electron microscopically NH<sub>4</sub>Cl-treated cells showed numerous enlarged lysosomes suggesting an accumulation of incompletely degraded cytoplasmatic material in the lysosomal compartment. We conclude that split products of lysosomal degradation of endogenous proteins are preferentially transported to the extracellular space via the basolateral plasma membrane.