Intraplatelet serotonin (5-HT) content was determined in 23 patients with type I (insulin-dependent) diabetes mellitus (IDDM), 23 patients with type II (non-insulin-dependent) diabetes mellitus (NIDDM), 29 patients with peripheral vascular disease (PVD) and 34 age-matched normal subjects. Intraplatelet 5-HT content in normal subjects showed an age-related decline (r = -0.45; P less than 0.008), as has been previously demonstrated. The median 5-HT content in platelets of the young normal subjects was 4.36 (range: 3.62-6.79) nmol 10(-9) platelets, while that in the elderly normal subjects was 3.87 (range: 2.8-6.0) nmol 10(-9) platelets and that in young + elderly subjects was 4.05 (range: 2.8-6.8) nmol 10(-9) platelets. The median intraplatelet 5-HT content was significantly lower (P less than 0.002) in IDDM patients: 3.0 (range 1.3-6.3), NIDDM patients: 2.5 (range 1.7-5.8), PVD patients: 2.42 (range 0.94-4.98) nmol 10(-9) platelets than that in all young + elderly healthy subjects. The presence of hypertension in DM patients caused a small but significant (P less than 0.05) decrease in intraplatelet 5-HT content, whilst its presence had no effect in PVD patients. In a smaller study, it was established that NIDDM and PVD patients have significantly (P less than 0.002) greater plasma 5-HT concentrations than controls. Insulin-dependent diabetes mellitus patients had greater plasma 5-HT concentrations but this did not achieve statistical significance despite a 66% increment in its value.(ABSTRACT TRUNCATED AT 250 WORDS)