The feeding of Aedes aegypti (L.) on blood and nectar is induced by phagostimulants: adenine nucleotides and sugars respectively. This work examines the responses of the four chemoreceptor cells in the labral apical sensilla to these phagostimulants. The apical chemoreceptors can detect the presence of adenine nucleotides. This part of the response is in good agreement with the gorging behavior. The output of the chemoreceptors cannot distinguish among different adenine nucleotides or among their concentrations (0.01-1 mmol/l), whereas gorging behavior is affected by the identity of adenine nucleotides and by their concentrations. Hence the gorging behavior cannot be driven by the output of these chemoreceptors alone. To the presence of adenine nucleotides Cell 2 was the only cell that responded with high frequencies, while the response of Cell 4 was almost abolished. The response of Cell 2 to ATP depended on the mosquito's physiological state. This dependence accorded well with the gorging behavior; Cell 2 responded with a higher frequency to ATP in the gorging state, than when not in a gorging state. The responses to sucrose and fructose constituted the only case recorded in which all these chemoreceptors failed to respond. This depression of response implies that other chemoreceptors must be present as sugar detectors.