To investigate the effect of S-availability on N and S assimilations, Enzymatic activity of nitrate reductase (NR), glutamine synthetase (GS), ATP sulfurylase (ATPS) and O-acetylserine(thiol)lyase (OASTL) was measured, and also followed by their activity staining in forage rape (Brassica napus L.) fed with three different SO₄²? levels (0, 0.1 and 2.0 mM) for 25 hours. NR activity decreased as S-availability decreased, especially in young and middle leaves. S-limited conditions (0 and 0.1 mM sot) resulted in a reduction of GS activity. The activity of ATPS tended to increase with decreasing the exogenous SO₄²? supplying level. The activity of OASTL also increased as S-availability decreased for all leaf tissues, while tended to decrease in root. The results indicated t㏊t N assimilation was restricted by the limited S-nutrition especially in the developing leaf tissues, but S assimilation was stimulated except for OASTL activity in the roots.