Dopamine (DA) D2/3R receptors (D2/3Rs) are of central interest in studies of the pathophysiology of schizophrenia. The sensitivity of D2/3Rs to DA can differ depending on the affinity state of D2/3Rs for DA. Early in vitro studies reported that D2/3Rs exist in two interconvertible affinity states for DA, referred to as high-affinity (D2/3high) and low-affinity (D2/3low) states. An increased proportion of D2/3high is assumed to correlate with dopamine hypersensitivity, suggesting a relationship with the psychotic symptoms of schizophrenia. Recently, several D2/3R agonist radioligands that are thought to bind preferentially to D2/3high in the living human brain have been developed. However, there are controversies regarding whether D2/3R agonist radioligands can be used to quantify only D2/3high in vivo. The findings of PET studies with D2/3R agonist radioligands indicate that various physiological conditions can modulate the affinity states of D2/3Rs. Given that the ratio of D2/3high to D2/3low fluctuates in various physiological states, a dual-radioligand approach using D2/3R agonist radioligands and a D2/3R antagonist radioligand to estimate the ratio of D2/3high to D2/3low is needed to detect alterations in D2/3high in schizophrenia patients.