Deprivation of dietary phosphate leads to a decrease in urinary excretion of ammonium in rats which may be mediated through an alteration in ammonium transport in the proximal tubule. In the present study the OK renal epithelial cell line, a model for the proximal tubule, was used to determine if NH4+ transport was changed during acute phosphate deprivation. The intracellular pH after perfusion with NH4Cl solution was used for calculation of intracellular NH4+ concentration. Intracellular [NH4+] increased linearly during the first 2 min of acidification with NH4Cl. NH4+ transport, defined by the initial rate of the increase in intracellular [NH4+], was decreased by 32% (P < 0.005) in phosphate-deprived cells. This transport process was inhibited by barium chloride, but not by DIDS, amiloride or ouabain, suggesting that the NH4+ transport pathway may utilize K+ channels. Acute phosphate deprivation may inhibit NH4+ transport in OK cells by decreasing membrane K+ permeability.