Computer simulation recently suggested that the prefrontal cortical circuit can perform fundamental cognitive operations of multi-target spatial working memory (Tanaka 2001 SFN #852.1, 2002a, b). The operations contain replacement, addition, rejection, and elimination. Moreover, slight deviation of the prefrontal cortical dopamine level in the normal range can switch these operations (Tanaka 2001, 2002a, b). The results suggest the involvement of dopamine in the oprations of multi-target spatial working memory. When the prefrontal cortical dopamine level is outside of the normal range, proper memory fields are not formed in the cortical neurons (Williams et al. 1995; Goldman-Rakic 2000; Yamashita and Tanaka 2002). This paper addresses how the wide change in the dopamine level affects the signal flow in the prefrontal cortical circuit by computer simulation of a circuit model. In the supraoptimal range of the dopamine level, inhibition in the circuit works effectively. But the circuit mechanisms of the inhi...