The stimulus integration time of lobster olfactory receptor cells in situ was determined using extracellularly recorded spiking responses from receptor cells and on-line high-resolution measurement of odor square pulses. At a fixed odor concentration, odor steps of 200 ms duration elicited maximum responses; shorter odor steps did not drive the cells to their maximum response and longer odor steps added spikes but did not result in higher firing rates. Excitatory processes peaked within 220 ms of stimulus onset. At 160­300 ms, stimulus intensity discrimination was most evident. Adaptation processes reduced response magnitude to near-zero levels within 1000 ms of stimulus onset. Olfactory receptor cells thus resolve odor peak onsets within the first few hundred milliseconds: this time window corresponds to the 4­5 Hz frequency of olfactory sampling (i.e. 'sniffing') as well as the rapid fluctuations in odor concentration that are common in natural odor plumes. The stimulus integration time of 200 ms may play a role in the filtering of information used by lobsters to orient to distant odor sources.