Behavioural responses of Triatoma infestans larvae to carbon dioxide and other odours of vertebrate origin were investigated in a locomotion compensator. T. infestans oriented towards airstreams enriched with carbon dioxide exhibiting a threshold response between 300 and 400 p.p.m. above the ambient CO(2) background. The accuracy of the oriented response to carbon dioxide improved with stimulus intensity. Remarkably, insects did not show any change in their sensitivity threshold to carbon dioxide with the starvation time. The attractiveness to carbon dioxide depended on the time of the day, i.e. these nocturnal bugs only oriented towards carbon dioxide-loaded airstreams during the first hours of the scotophase. L-lactic acid did not evoke oriented responses when it was presented as a single stimulus in a wide range of intensities. However, a marked synergism was evident when L-lactic acid was combined with a sub-threshold concentration of carbon dioxide. Under this condition, the threshold response to carbon dioxide decreased to 75-150 p.p.m. above ambient CO(2) background. The isomer D-lactic acid evoked no response, either alone or in combination with carbon dioxide. When insects were stimulated with 1-octen-3-ol a significant positive orientation was found. This response was not modified by the addition of carbon dioxide.