Ab initio calculations (HF, MP2, DFT for isolated and PCM for solvated molecules) were performed for cytisine ( 1) and its model compounds: N-methyl-2-pyridone ( 2) and piperidine ( 3). Among three heteroatomic functions ( carbonyl oxygen, pyridone and piperidine nitrogens) considered as the possible sites of protonation in 1, surprisingly the carbonyl oxygen takes preferentially the proton in the gas phase whereas in water the piperidine nitrogen is firstly protonated. For model compounds, the piperidine nitrogen in 3 is more basic than the carbonyl oxygen in 2 in both, the gas phase and water.