The spontaneous activity of guinea-pig renal pelvis-ureter is regulated by the adrenergic system. Spontaneous rhythmic contractions, and contractions induced by Noradrenaline are inhibited by Dihidroergotamine and Phentolamine. Alpha-adrenergic blocking agents block also contractions induced by histamine, angiotensin and barium chloride, but not contractions induced by electric stimulation. The Authors suggest an hypothetical model for the activation of the adrenergic receptor: Noradrenaline (NE) recognition sites are activated only by NE, whereas complementary sites can be activated by NE or other agonists. Both sites are blocked by alpha-adrenergic blocking agents.