The effects of lithium on membrane function and cellular processes are considered in relation to lithium's physicochemical properties. Several cellular regulatory functions, including cyclic AMP formation and metabolism and biogenic amine neurotransmitter metabolism, are examined as possible mechanisms whereby lithium might exert its mood stabilizing actions. The necessity for understanding lithium's interactions with “receptors” – the molecular sites affected by lithium – in considering its pre- and post-synaptic effects is stressed. * Paper read at the WPA meeting on “Long-term pharmacological treatment in psychiatry”, 26th–28th November, 1975, Acapulco, Mexico.