Results from a longitudinal study (blood drawn at days 29, 64, 89,124, 142, and 182 of the protocol) shows that the concentration of platelet-poor plasma (PPP) methionine(5)-enkephalin (MET) in healthy, drug-free, white male individuals (n = 5) remains within a relatively narrow range, well within the experimental error of the analytical procedures used. Interindividual differences fail to reach statistical significance [x +/- SD and range (MET picograms per mL of PPP) of 91.2 +/- 15.1, 67.1 +/- 113.5; 69.6 +/- 7.5, 66.1-90.1; 76.6 +/- 12.6, 58.5-93.1; 86.8 +/- 10.9, 76.3-107.4; and 84.5 +/- 11.4, 68.9-103.4; for subjects 1-5, respectively]. MET levels were similar to those recorded from single samples obtained from a group of 24 white male, age-comparable, drug-free healthy volunteers [x +/- 6 SD and range (picograms of MET per mL of PPP) of 83.3 +/- 15.1 and 57.4-119.1]. The controls' range for all the subjects (n = 29) was 57.4-119.1 pgMET/mL PPP. Compared with the controls, individual patients with cluster headache (CH) show a much wider variation in PPP MET levels (blood drawn at different time intervals, at least 10 samples per patient, over a period of 221-298 days), with many (slightly over half) of single values below the controls range; no single MET level was above the controls range [x +/- SD and range (picograms of MET per mL of PPP) of 56.4 +/- 27.7, 6.1-100.5; 72.6 +/- 20.5, 43.0-113.0; 46.0 +/- 28.5, 10.0-92.6; 53.6 +/- 27.5, 13.0-101.0; 52.0 +/- 26.1, 17.5-83.6; 63.5 +/- 22.3, 21.7-91.3 for individuals A-F, respectively]. Although interindividual differences within the patients' group were not statistically significant, their peptide levels were significantly lower than those of controls. Neither the presence of unspecified "headaches between clinic visits'' and "daily headaches" (patients E and F, respectively), nor the use of a number of drugs known to lack inhibitory activity upon the aminopeptidase-MET degradation reaction, seemed to significantly influence MET concentration. The results could lead to a better understanding of the etiology of the pain associated with CH, with the relative changes in plasma peptide perhaps reflecting the patients' vulnerability to such a condition. Pharmacological modulation of MET function may prove useful in the treatment of CH-associated pain, whether the development of such drugs could find useful pharmacological applications remains to be explored.