Drugs targeting the opioid neurotransmission system have been used for centuries recreationally and for medical purposes. In spite of this vast experience and competence in opioid pharmacotherapy, fine details about the cellular and molecular mechanisms underlying opioid receptor physiology remain unknown. We present here two methods with single-molecule sensitivity, confocal laser scanning microscopy with avalanche photodiode (APD) detectors (APD imaging) and fluorescence correlation spectroscopy (FCS) suitable for nondestructive study of molecular interactions and intracellular transporting processes in living cells. These high-resolution methods provide functional readouts, giving measures of concentration, mobility and affinity, for the investigated molecules and enable us to monitor changes in these properties in living cells in real time. We have used these methods to study early events in opioid receptor activation with specific and nonspecific ligands, and discuss the new insights obtained by these approaches.