Optical signals were recorded in the in vivo rat piriform cortex (PC) in response to olfactory bulb electrical stimulation. Sometimes the early response was followed by a longer latency component with an occurrence probability of 0.25. In order to compare the early and late activity, the ratio between early (disynaptic) and late wave amplitudes was measured at each recording site on the whole PC. Its spatial distribution revealed that the relative importance of the late activity was larger in the most posterior part of the PC whereas the late wave was rarely observed in the anterior PC. Such a result gave new information on the functional heterogeneity of the PC.