The transport of ammonia in the proximal tubule is a complex interaction of a number of processes. Ammonia transport in the proximal tubule is clearly bidirectional; ammonia is secreted into the early proximal tubule lumen, but later in the proximal tubule, efflux out of the lumen may result in net ammonia reabsorption. Two mechanisms of ammonia transport have clearly been established: NH3 diffusion and NH4+ transport on the Na(+)-H+ exchanger. The relative contribution of these pathways to ammonia transport is still unsettled. Other pathways for ammonia transport, particularly NH4+ efflux out of the lumen, may be important as well. A variety of factors may modulate ammonia transport: plasma, cell and luminal pH, luminal flow rate, luminal potassium, and angiotensin II. Each of these factors also alters ammonia production rates and in most circumstances, ammonia transport appears to follow ammonia production rates.