Objective “Lie flat” has recently become a buzz word describing a specific psychological state in some individuals. However, its psychological meaning and mechanisms remain unclear. Lack of motivation undercertain conditions may be one of the key psychological characteristics of this condition. Methods Sixteen male SD rats were randomly assigned to two food-restriction levels (90% and 80% of baseline weight, respectively), and subjected to a sucrose-pellet self-administration task.The establishment of the model is divided into three stages. The rats were trained to self-administer sucrose pellets in a high-reward learning stage and in a low-reward learning stage. We then assessed “lie flat” behavior in a preference-test stage, in which rats could choose between high-reward- and low-reward-paired nose-pokes in a discrete choice procedure. Results Combining self-feeding training with Western blot, the Results showed that: (1)rats showed significantly decreased nose-poke behavior for low-reward, i.e. “lie flat” behavior, but not for high-reward, when ≥90% of the maximal reward was obtained in the high-reward task and the effort (X) in the low-reward task increased to the breaking point. (2)Dopamine D1R expression in the NAc was significantly higher in the “lie flat” group compared with the “not lie flat” rats, while D2R expression was similar in both groups. Conclusions Rats can show “lie flat” behavior , and up-regulation of D1R expression in the NAc may be a key part of the molecular basis responsible for this motivation deficiency in “lie flat” behavior. These Results extend our understanding of “lie flat” behavior, and provide a new paradigm for the study of its mechanism.