SUMMARY1. 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 μmol/1 (s.d. = 1.3); guanidinosuccinic acid 0.66 μmol/1 (s.d. = 0.22); guanidinoacetic acid 7.06μmol/l (s.d. = 1.29); creatine 80.9μmol/l (s.d. = 37