Bitterness of naturally occurring substances seems to be due to a balance between the "bitter unit" and the hydrophobic portions of the molecule. The study based on the Rabdosia diterpenes indicates that the bitter unit consists of a proton donor DH group and a proton acceptor A group. In addition to this DH-A unit, an A-A unit may also be possible. The bitter compounds could enter the molecular structure of the receptor site with the bitter unit oriented into the aqueous phase by hydrogen bonding and the hydrophobic portion aligned into the lipid phase by dispersion forces.