Albumin and fibrinogen synthesis appear to account for the majority of protein exported by the liver and therefore make a substantial contribution to that of whole-body protein synthesis. However, data on the protein synthetic rates of albumin and fibrinogen in normal subjects are limited. Albumin and fibrinogen synthetic rates were measured simultaneously over a 120-min period in normal subjects (n = 6) by using a flooding dose of 2H5-phenylalanine. Tracer incorporation into proteins was measured by gas chromatography/mass spectrometry. Body mass index, circulating concentrations of insulin, albumin, fibrinogen, C-reactive protein, and plasma volumes of the subjects were all within the normal reference range. There was a small and transient rise in circulating insulin concentrations following the flooding dose of phenylalanine. The median fractional synthetic rate and absolute synthetic rate for albumin was 10.3%/d and 208 mg.kg-1.d-1, respectively. The median fractional synthetic rate and absolute synthetic rate for fibrinogen was 19.5%/d and 28 mg.kg-1.d-1. In the context of the current interest in manipulating the inflammatory response of patients with various disease states, we introduce the concept of an acute phase protein quotient (APPQ). The APPQ is defined as the absolute rate of fibrinogen synthesis divided by that of albumin. In this group of normal subjects, the median APPQ was 0.14.