Human pain responsivity was defined as the subject's behavioral pain endurance time (PET) to the 1 +/- 0.3 degrees C cold-pressor test, a naturalistic and clinical analogue tonic pain model. Over the past 2 years, we have consistently observed a behavioral dichotomy of pain responsivity in each of our 6 studies (all at P less than 0.000001 effect level), totaling 205 subjects. Overall, the pain-tolerant (PT) subjects could endure the whole 5 min (note that 3 min was the ceiling criterion in the last study) of cold-pressor test, while the pain-sensitive (PS) subjects could merely tolerate the test for an overall mean of 60 sec, 20% of PET in the PT group. No overlapping of distribution was observed between these 2 populations. Further, we observed that the percentage of subjects in each of these 2 groups varied substantially across studies. The mean pain perception (Visual Analogue Scale) of tonic pain ranged from 60-70 for both aversiveness and intensity scales. The characteristics of this tonic pain, assessed by the McGill Pain Questionnaire (MPQ), showed similar patterns across each study with a high degree of consistency. Although ratings of pain aversiveness did not differ in the PT vs. PS subjects, ratings of pain intensity did differ, with the PT subject reporting less pain. It was found that state anxiety correlated with MPQ scores for PS, but not PT, subjects. Additionally, psychological tests (Tellegen Absorbance Scale, Kleinknecht Fear, Spielberger Trait-Anxiety) were positively correlated with certain MPQ measures for PS, but not PT subjects. Multivariate regression analyses indicated, in the PS but not the PT group, that 36% of variance in pain score (MPQ-T) could be predicted by the psychological trait factors. The general level of fear contributed singularly as the major predictor variable in the pain-sensitive individuals. We consider this tonic pain model indeed offers a succinct empirical paradigm to study human pain responsivity in general. The psychological/physiological etiology of such drastic human pain responsivity requires intense systematic investigations. This report discusses the results in: (a) individual differences in pain responsivity, (b) characterization of the cold-pressor test as a model for tonic pain, (c) contrast between PS and PT groups of pain perception and state anxiety, and (d) psychological determinants of measures for cognitive, perceptual and affective domains. Discussion was also focused on the experimental tonic pain model and its generality for clinical pain, as well as the basic model of the cold-pressor test for human tonic pain responsivity.