This study, performed on the urinary bladder of Bufo marinus, was to investigate the characteristics of H+ excretion in the normal and metabolic acidotic toad. Experiments were run in modified Ussing chambers in the presence and absence of exogenous CO2. Amiloride in the mucosal medium (1.5 X 10(-4) M) inhibited H+ excretion in the acidotic bladder but this inhibition was reversed in the presence of 5% CO2. Na+-free mucosal medium revealed a component of H+ excretion in the normal toad that is Na+-dependent and not reversed by 5% CO2. Acetazolamide (10(-3) M) had no effect on H+ excretion in normal toad but inhibits excretion in the acidotic toad both in the presence and absence of exogenous CO2. In both the presence and absence of exogenous CO2 and in the presence of varying pH gradients, the bladders from acidotic toads excreted H+ at a greater rate and were able to generate greater pH gradients than in normal bladders. Voltage clamp experiments revealed that H+ excretion in the acidotic toad was inhibited by a mucosal potential difference of -20 to -100 mV but this inhibition was completely reversed by 5% CO2. In the normal toad bladder H+ excretion was stimulated by a mucosal potential difference of -20 to -100 mV in both the presence and absence of exogenous CO2. Our evidence suggests the possibility of two H+ excretory mechanisms in toad urinary bladder each with its' own set of characteristics dependent on the acid-base state of the animal. Proposed models are presented for these mechanisms.