1. A cation-exchange-chromatographic method for the determination of methylguanidine in serum is described.
2. In ten normal subjects, the mean serum methylguanidine concentration was 0.055 (SE 0.019) mg/100 ml and in ten uraemic patients it was 0.175 (SE 0.038) mg/100 ml. This difference is significant (P < 0.02).
3. Recent claims that methylguanidine is present in uraemic serum in much higher concentrations are shown to be due to artifactual conversion of creatinine when methods involving charcoal chromatography are employed.
4. The results of intoxication experiments, in which blood concentrations of methylguanidine similar to those found by charcoal-chromatographic methods have been reproduced, must be interpreted with caution.