Two polymethacrylamides bearing glycylglycine (PGlyGly) and phenylalanine (PPhe) in the side chain were synthesized by free radical polymerization of the corresponding monomers (general formulas : -[CH2-C(CH3)-CO-NH-CH2-CO-NH-CH2-COOH]-n and -[CH2-C(CH3)-CO-NH-CH(COOH)-CH2-C6H5]-n, respectively). PGlyGly behaves as a normal polyelectrolyte in relation with its hydrophilic nature. On the contrary PPhe takes a compact conformation in water. The free enthalpy of stabilization of this conformation is high as revealed by potentiometric titrations. The influence of some additives (metal ion, urea, organic solvent) on the compact conformation and the pH-induced conformational transition was investigated. It was concluded that the main factor responsible for the existence of this structure consists of hydrophobic domains with nonpolar interactions between the aromatic side chains.