Thin slices, 500μm in thickness, prepared from the guinea pig olfactory cortex and maintained in vitro, generate presynaptic and postsynaptic potentials following electrical stimulation of the lateral olfactory tract. These responses, however, cannot be elicited immediately after slice preparation;they start to develop gradually as the incubation continues. It was found that there was a remarkable time lag for the development between the pre- and the postsynaptic potentials. The presynaptic potential appeared early and developed its maximum height within 10 to 20 min from the onset of incubation. This time course was only slightly influenced by low temperature. Development of the postsynaptic potential in incubation was delayed and the time required to obtain the maximum height was about 30 min at 37°C and 70 min at 22°C. Upon step-like warming from 24°C to 37°C, there was a rapid increase in the amplitude of the response. In the glucose-deficient solution the presynaptic potential maintained its amplitude longer than the postsynaptic potential. Frequency potentiation of the postsynaptic potential was studied by applying a train of six stimuli. The maximum augmented ratio among six responses was the highest in early incubation and reached a steady at 50min. The ratio of the post-tetanic potentiation of olfactory neurons was the highest at 5 min incubation and reached a steady at 20 min.