1 The temporal changes in constitutive NO-synthase (cNOS) and in calcium-independent NO-synthase activities were studied in mice subjected to 2 h of transient focal cerebral ischaemia. The changes in brain nitrites/nitrates (NO x ) content were also studied. 2 NOS activities were measured by the conversion of L-[ 14 C]-arginine to L-[ 14 C]-citrulline. Brain NO x contents were investigated by the Griess colourimetric method. 3 cNOS activity in the infarcted cortical area was signi®cantly reduced after 6 h of reperfusion and this activity remained attenuated for up to 10 days after ischaemia. A calcium-independent NOS activity began to increase 48 h after reperfusion, reached a maximum at 7 days and returned to baseline at 10 days. 4 There was a signi®cant increase of brain NO x content beginning after 3 days of reperfusion. This increase was maximal at 7 days and returned to baseline at 10 days. 5 Thus, ischaemia followed by recirculation leads to a rapid, prolonged drop in cNOS activity in the infarcted cortex. There is also a substantial appearance of calcium-independent NOS activity in the later phase of transient ischaemia, leading to an important increase of NO x production.