Three groups of receptors defined pharmacologically by means of specific agonists and competitive antagonists were described: receptors for the so-called gastrointestinal hormones, receptors for the autacoids and receptors for the neuromediators. Together with the better known post-synaptic receptors pre-synaptic ones were also discussed in the light of the different possible interactions between one and the other. The mode of action of drugs stimulating (agonists) or inhibiting (antagonists) the same receptors is so far a very intriguing problem. The availability of newer and more selective compounds capable of distinguishing not only different kinds of receptors but also the subtypes of a single class of receptors will certainly contribute to better understanding of this situation.