Trypsinogen was identified in cerebrospinal fluid (CSF), where it has not previously been reported and its activation state in experimental subarachnoid haemorrhage (SAH) in rats and in neurosurgical patients was determined. Trypsinogen activation peptide (TAP) release provided an equimolar marker for trypsinogen. Total TAP was significantly reduced to 26% of the baseline level (P<0.02) following experimental SAH in 15 rats but not in ten sham operated controls (P=0.3). TAP was also measured in patients with ruptured (n=11) and unruptured (n=9) aneurysms who underwent craniotomy to clip an aneurysm. Postoperatively there was a significant fall in TAP concentration (P<0.005) in both groups. Trypsinogen, as identified by CSF levels of TAP, is activated by SAH in rats and by craniotomy for aneurysmal clipping in patients.