Brief CO2 laser radiant heat pulses activate both A delta- and C-fibres. In the evoked potential (EP) late and ultralate components can be seen as correlates of first and second pain. Usually the ultralate EP appears to be suppressed. It could be uncovered by a preferential A-fibre block, and in two neurological patients with tabes dorsalis and with a polyneuropathy involving myelinated fibre loss. Due to a strong latency jittering the shape of the ultralate component is distorted in the conventional average. Latency corrected averaging, adaptive filters or parametric spectral estimators are needed to analyze these EP components. As a result the filtered ultralate waveforms look very similar to the late EP components. Clinical application of CO2 laser EPs promises to nonivasively assess A delta- and C-fibre function.