Albumin synthesis was calculated in healthy male volunteers consuming diets differing in the relative contribution of protein from animal or vegetable sources. In one study (Study 1, n = 4) two isoenergetic and isonitrogenous diets were consumed for a period of 10 d each. One diet (diet A) was animal protein rich (74%), the other one (diet V) contained 67% of vegetable protein. Albumin synthesis rate was measured from L-[(2)H(5)]phenylalanine incorporation (43 mg/kg) at the end of each dietary period. Both albumin fractional synthesis rate (FSR) (5.7 +/- 0.6 vs. 6.7 +/- 0. 8%/d, P = 0.04) and absolute synthesis rate (ASR) (123 +/- 6 vs. 143 +/- 8 mg. kg(-1). d(-1), P = 0.05) were reduced after diet V. In a second study (Study 2, n = 8) a third dietary treatment was added (Diet VS). This was similar to diet V but supplemented with soy protein (18g/d). The results of study 2 confirmed that albumin synthesis was reduced after diet V (FSR: 5.9 +/- 0.3 vs. 6.7 +/- 0. 5%/d, P = 0.015; ASR: 126 +/- 7 vs. 146 +/- 9 mg. kg(-1). d(-1), P = 0.007), but it also showed that the drop could be prevented by adding supplemental protein to the predominantly vegetarian diet (Diet VS) (FSR: 6.4 +/- 0.3%/d, P = 0.08; ASR: 140 +/- 7 mg. kg(-1). d(-1), P = 0.03). Albumin synthesis appears to be modulated by changes in the proportion of animal vs. vegetable protein occurring in the diet. The mechanism might be related to differences in digestibility and consequently in net amino acid availability between diets.