Many physiological mechanisms show a circadian (around 24 hours) rhythm, related to adrenal, renal, cardiovascular function and body tem-perature (Conroy & Mills 1970). The best-estab-lished rhythm is body temperature. The ability of healthy volunteers to withstand painful electric shocks has a documented circadian rhythm, the subjects becoming more sensitive as the day pro-gresses (Procacci 1972). The present study assesses variability in intensity during the day of continued pain of pathological origin and relates changes in intensity to other factors (Blake 1967, 1971, Folkard 1975).

As pain is essentially a subjective phenomenon the patient's own assessment of intensity was recorded. This meant assessments were made only when the patient was awake so that the study is one of diurnal variation (variation over the waking day). Subjective alertness is also known to exhibit a circadian rhythm: it is possible that the intensity of pain experienced by a patient might be related to how drowsy he felt. Finally, it is clearly impor-tant to determine whether the diurnal variation of the intensity of pain, if it exists, differs in easily identifiable facto! s.