The aim of the present work was to study the effect of ammonia and lactate on growth, metabolism, and productivity of BHK cells producing a recombinant fusion protein. Results show that cell growth was reduced with the increase in ammonia or lactate: k(1/2) of 1.1 mM and 3.5 mM for stirred and stationary cultures, respectively, for ammonia and of 28 mM for both stationary and stirred cultures for lactate, were obtained. The cell-specific consumption rates of both glucose (q(Glc)) and glutamine (q(Gln)) increased, whereas that of oxygen (q(O2)) decreased, with the increase in ammonia or lactate concentrations. The cell-specific production rates of lactate (q(Lac)) increased with an increase in ammonia concentration; similarly for the cell-specific production rates of ammonia (q(Amm)), which also increased with an increase in lactate concentration; on the other hand, both q(Lac), and q(Amm) markedly decreased when lactate or ammonia concentrations were increased, respectively; lactate was consumed at lactate concentrations above 30 mM and ammonia was consumed at ammonia concentrations above 5 mM. In vivo (PNMR)-P-31 experiments showed that ammonia and lactate affect the intracellular pH, leading to intracellular acidification, and decrease the content in phosphomonoesters, whereas the cell energy state was maintained. The effect of lactate on cell growth and q(Gln) is partially due to osmolarity, on q(Glc) and q(Amm) is entirely due to osmolarity, but on q(Lac) is mainly due to lactate effect per se. An increase in ammonia from 0 to 20 mM induced a 50% reduction in specific productivity, whereas an increase in lactate from 0 to 60 mM induced a 40% decrease.