Whether ageing is associated with increased fibrinogen concentration and production remains unclear. We measured fibrinogen fractional (FSR) and absolute synthesis (ASR) rates in male volunteers, of either young (mean age: 28 years, range: 22–34) or middle age (mean age: 57 years, range: 38–72), using a leucine-tracer isotope dilution technique. In the middle-age group, neither fibrinogen FSR (20.8 ± 1.6%/day) nor ASR (1.8 ± 0.1 g/day), or concentration (274 ± 15 mg/dl), were different from those of the younger group (FSR: 20.2 ± 1.4; ASR: 1.7 ± 0.2; concentration: 265 ± 8, respectively). Leucine Ra, an index of endogenous proteolysis, was ≈20% lower in the older than in the younger group (P < 0.02). Thus, middle age in males is not associated with increased fibrinogen concentration and turnover, whereas endogenous protein breakdown in decreased. Factor(s) different from age per se are likely to be involved in the dysfibrinogenemia possibly occurring with ageing. Protein turnover is already reduced in middle-age males.