Circadian (24 hour) rhythms are the manifestation of the output of the hypothalamic, thalamic and neuroendocrine structures, which are driven by the central pacemaker or 'clock' and cause a diurnal variation in the levels of activity of body systems. This research was carried out to explore skin sensory thresholds in relation to time of day. Sensory thresholds of skin were assessed in the morning (~09:00 hours) and late afternoon (~17:00 hours) in normal human subjects, using subjects as their own controls. Touch-pressure (T-P) thresholds were tested with calibrated aesthesiometry filaments in glabrous and hairy skin areas. Pin prick and heat pain thresholds of glabrous skin were also tested, using adapted aestheiometry filaments and a small heat probe, respectively. Axillary temperatures were recorded. In addition, further T-P and pin prick thresholds tests were made at other times through the day / evening. Aesthesiometry tests were made under varying experimental conditions, and also in a group of pre-disease subjects (permanent night-shift workers). A significant, robust pattern of diumal variation was noted in both T-P and pin prick pain thresholds in normal subjects, in all experiments, with ~60% change between morning and late afternoon / early evening tests, although the magnitude of change was, to some extent, site dependent. The observed pattern of change also indicated that both T-P and pin prick thresholds follow a circadian rhythm, with maxima (least sensitivity to stimulus) at around 07:00 hours and minima (greatest perceived sensation) at around 19:00 hours, i.e. in anti-phase to the rhythm of core body temperature. However, heat pain thresholds did not show diurnal variation, although subjects demonstrated reliability of criteria and good discrimination. As diurnal variation in T-P thresholds was lost in pre-disease (night-working) subjects, it suggests that T-P threshold sensation could be used as a marker of the activity of the circadian timing system. The influence of the output of the circadian timing system on the somatosensory system is discussed, and a model of the inter-relationship of the two systems is presented.