2-Amino-4-hydroxy-1, 4, 5, 6-tetrahydropyrimidine (I), the cyclic guanidine moiety of tetrodotoxin, and its derivatives were prepared to examine their chemical and physical properties in comparison with the toxin and its derivatives. 1. Oxidation of I with chromium trioxide-pyridine gave β-alacreatinine (III). 2. Etherification of I with alcohols in the presence of acid catalyst afforded the corresponding 4-alkoxy derivatives (IV and V). 3. Acetylation of I under various conditions gave N-acetyl derivative (VI), O-acetyl derivative (VII), and N-acetyl-1, 6-dihydro derivative (VIII) respectively. 4. Interconversion of the acetyl derivatives is also described.