The term neurohumoral transmission designates the transfer of a nerve impulse from a presynaptic to a postsynaptic neuron by means of a humoral agent e.g. a biogenic amine, an amino acid or a peptide. This process involves several steps, i.e. biosynthesis, storage, release, receptor interaction and inactivation of the transmitter. A neuromodulator modifies, for instance the release of a transmitter by action on a presynaptic transmitter neuron. Biogenic amines may also be released from non-synaptic nerve terminals and possibly exert a modulatory effect on other neurons. The regulation of the transmitter dynamics at the enzymatic level occurs by end-product inhibition and by enzyme induction. In addition, there are regulatory mechanisms originating from autoreceptors and postsynaptic receptors which operate via feedback action. Stimulation or inhibition of receptors may lead to receptor sub- and supersensitivity respectively. Numerous neuro-psychotropic drugs used in medical practice act on the various steps of neurohumoral transmission and thereby influence the dynamics of neurotransmitters and modulators.