Ingestion of meals containing cooked meat caused a marked postprandial increase in serum creatinine concentration, whereas ingestion of raw meat had no effect on serum creatinine. A peak mean value was seen 3 hours after the start of meals. No change in creatinine clearance was observed. Analysis of serum creatinine by conventional autoanalyser and by an ion exchange method ('true creatinine") showed a high degree of correlation, making a possible effect of non-creatinine chromogens unlikely. It was shown that creatinine is produced from creatine when meat is boiled.