Larval mosquitoes including Aedes aegypti have phenomenal pH regulatory abilities, maintaining hemolymph pH within narrow limits in waters ranging from pH 4 to pH 11 (Clark et al. 2004). In acidic water (pH 4), larval Aedes aegypti regulate hemolymph pH by substantially increasing rates of drinking and excretion. Chronic exposure to acidic water causes an increase in Malpighian tubule mitochondria suggesting increased energy demands of transport mechanisms. The increase in drinking and excretion rates are hypothesized to allow increased acid clearance by reducing the electrochemical gradients opposing H+ excretion in the Malpighian tubules. Alaproclate, a serotonin-selective reuptake inhibitor, causes hemolymph alkalosis, increases survival in acidic water, and is toxic in neutral and alkaline water. Acid/base homeostasis in acidic media thus appears to be mediated by serotonergic stimulation of Malpighian tubule fluid and acid secretion. In highly alkaline media (pH 11), larvae do not appear to either excrete base or absorb acid. Instead, they maintain hemolymph pH homeostasis while excreting fluid more acidic than their hemolymph. This strategy is apparently made possible by reducing drinking rates to a minimum, and by generation of sufficient metabolic acids to neutralize ingested base.