A survey of the available data on the properties of cyclopropane systems shows that the common assumption that cyclopropane can conjugate with unsaturated systems is questionable. A series of phenylcyclopropanecarboxylic acids, and a series of phenylcyclopropylamines were prepared, and a study was made of the basicity and nucleophilicity of the latter, and reactivity of the former, and the spectral properties of both. It was shown that the lone pair of the nitrogen of a phenylcyclopropylamine does not appear to conjugate with the cyclpropane ring, that the cyclopropane ring cannot transmit mesomeric effects, and that the cyclopro-pane ring electrons are not appreciably more polarizable than is expected classically. A general explanation of the properties of cyclopropane systems is offered in terms of strain, and it is pointed out that in certain cases, particularly in consideration of polarizability, the effects of ring closure cannot be neglected. It is concluded that the cyclopropane ring does not conjugate with unsaturated centres.