A pair of antagonistic thermoreceptive cells is associated with each of two peg-in-pit sensilla located on the antennal tip of Aedes aegypti. One, the warm cell, responds to rapid warming with a sudden increase in the rate of discharge. The other, a cold cell, responds to rapid cooling with a sudden increase in the discharge rate. When temperature changes are provided by oscillating changes in the convective heat contained in the stimulating air stream, the oscillating discharge rates of both cell types are in advance of the oscillations in temperature and slightly behind the oscillations in the rate of temperature change. Analysis of these phase relationships shows that both cell types respond not only to the actual temperature at particular instants in time (instantaneous temperature) but also to the rate with which temperature changes. Individual responses are therefore ambiguous and signal tendencies rather than precise instantaneous values. When the temperature oscillations are delivered by changes in radiation power, however, the oscillating discharge rates of the warm and cold cells are in step with the oscillations in temperature. Here, individual responses signal instantaneous values of temperature rather than tendencies. The power of radiant heat required to modulate the discharge rates is relatively high, suggesting that infrared radiation is not a significant cue in distant host location.