Carbon fibre filled microelectrodes were used for extracellular recording of spikes discharges from 345 neurones of the lateral aspect of the left olfactory bulb of the rat under pentobarbitone anaesthesia. The identity of the neurones was determined by antidromic activation of the cell axons by electrical stimuli applied to the lateral olfactory tract (LOT) using collision blocking and twin stimuli. The bulbar units were classified as mitral (269) or tufted (16) cells; units in a third class were not identified (60). Application of stimuli (21) was made over a period of 15 s while slowly increasing the odorant concentration monitored by a flame ionization device (FID). The stimulating efficiency reached a maximum at a nasal flow of about 8 cm3 s-1. The response type excitation was seen in 12.0% and inhibition in 11.8% of the 7245 odour presentations. Each response type was subdivided into four classes according to the reactions to ramp stimulation. In the majority of cases the spike discharge remained constant or followed the odour concentration for the entire stimulation period. In 18% of the cases, the response adapted quickly towards the spontaneous activity level.