Concentration of guanidines in normal human plasma

1. The concentration of methylguanidine and guanidinosuccinic acid has been measured in normal human plasma and discrepancies between various published methods for measuring methylguanidine have been resolved. Previous methods have been modified to allow these guanidines, together with guanidinoacetic acid and creatine, to be measured on the same serum sample. 2. Mean values in six normal subjects were as follows: methylguanidine 1.62 mumol/1 (s.d. = 1.3); guanidinosuccinic acid 0.66 mumol/1 (s.d. = 0.22); guanidinoacetic acid 7.06 mumol/1 (s.d. = 1.29); creatine 80.9 mumol/1 (s.d. = 37.9).