This paper studied the NOR activity and Ag-AA of a girl with bisatelliled extra small chromosome (E.S.C)as compared with that of her siblings. The results indicate that(1)the girl with E.S.C and her siblings without E.S.C were shown to have similar frequencies of Ag-AA (P>0.25); (2) the girl with E.S.C have significantly higher frequencies of NOR activity than her siblings(P<0.025); (3) the NOR activity frequencies not including E. S.C or including only oae arm end of E.S.C were similar among the girl and her siblings(P>0.25; P>0.5).It is suggested that ( 1 ) the NOR activity shows incomplete Compensatory effect in NOR tetrasomy

but shows complete compensatory effect in NOR trisomy; (2) a regulatory range may be present in the compensatory mechanism for NOR activity