1. 3-Methylbutanal is a normal constituent of human plasma and is elevated in patients with hepatic encephalopathy. Therefore these studies examined the possible source and site of synthesis of 3-methylbutanal and its effect on the central nervous system of adult rats.
2. 3-Methylbutanal was found to be a normal constituent of rat plasma and increased two to five times when leucine comprised 5% of the diet. Neomycin in the diet prevented the leucine-induced rise in plasma 3-methylbutanal. When this was injected at a dose of 120 mg/kg abnormal EEG patterns and sleep-like behaviour occurred, whereas smaller amounts (30 mg/kg) increased brain serotonin concentrations.
3. 3-Methylbutanal is a normal component of rat plasma: it may be derived in part from colonic bacterial breakdown of leucine and may influence central nervous system function. A possible relationship of 3-methylbutanal to the pathogenesis of hepatic encephalopathy is suggested.