The effect of changes in temperature on the activity of thin intracortical unmyelinated fibers was investigated in rat hippocampal slices. The amplitude of the presynaptic volley was increased by 40% when the temperature was raised from 23 degrees C to 30 degrees C, whereas the area of the presynaptic volley was reduced. Increasing the temperature above 30 degrees C was without further effect on the amplitude. The conduction velocity was 0.3 m.s-1 at 35 degrees C and 0.1 m.s-1 at 12 degrees C. The Q10 of velocity (25 degrees C-35 degrees C) was 1.6.