A host-guest system involving derivatives of 2-amino-4(3H)-pyrimidone and creatinine was developed. 1H NMR and 13C NMR experiments give evidence of complex formation. A geometry optimization with Gaussian 88 suggests a complex structure in which the 5-membered and the 6-membered heteroring are connected by one long and two shorter hydrogen bonds. The hosts described strongly enhance the extraction of creatinine from its aqueous solution into CH2Cl2 and CDCl3. As the creatinine concentration in the organic solvents may be determined by measuring the changes in the UV spectrum of the hosts upon complexation, derivatives of 2-amino-4(3H)-pyrimidone may eventually be used in optical creatinine sensors.