The purpose of this study was to investigate distinctive natures of the mentally retarded children for acquisitional mechanisms of the eyelid conditioning, through the effects of US upon organisms. In equal-CA comparisons, fifteen mentally retarded children and normal children in the same junior high school were conditioned by CS as a light and US as an air puff: eyelid responses were recorded by the electromyogram. Results were as follows:(1)For CRs, in comparison with the normals the inferiority through the acquisitional period for the retarded children was suggested.(2)For URs, the retarded subjects did not from stable URs to the air puff in comparison with the normals. Then, to investigate the natures of the mentally retarded children in the acquisitional process of the eyelid conditioning, recordings for movements of the eyelid responses were analized. The main findings were as follows:(a)In the mentally retarded children, a marked trend was shown: the significant eyelid responses did not occur immediately from the onset of the US.(b)The lack of concentration in the CS for the mentally retarded children proved to be higher. Finally, the following points were discussed.(1)For two groups, the US did not produce the same effects on organisms[c.f., Results(a)].(2)Results(a)and(b)were factors regulating the occurences of CRs at a lower level.