A cellular suspension from rat submandibular glands was prepared with collagenase. The intracellular pH (pH i) was estimated with 2′, 7′-bis-(2-carboxy-ethyl)-5 (6)-carboxyfluorescein (BCECF). After exposure to NH 4 Cl, the pH i transiently increased (diffusion of NH 3) and then dropped (influx of NH 4+). Isoproterenol increased 2.5-fold the rate of NH 4+ influx; bumetanide, an inhibitor of the Na+-K+-2Cl−-cotransporter blocked the response to isoproterenol, confirming that the beta-adrenergic agonist stimulated the cotransporter. Forskolin (1 μmol/L) mimicked the response to isoproterenol. VIP (1 nmol/L-1 μmol/L) also increased the activity of the cotransporter. Cyclic AMP rather than calcium was the mediator of this activation since 1) carbachol which increased the [Ca 2+] i fivefold increased the uptake of NH 4+ by only 50%; 2) only high concentrations of VIP …