http://www.icp.ucl.ac.be/mexp/vipg_blt.html

PROTOCOL
Anesthesize a mouse
Make a standard incision on the dorsal tail vein using Simplate blades
Immerse the tail in NaCl 0.9% at 37.5ѓC
Record the time from the moment blood is observed to emmerge from the wound until cessation of blood flow

ADDITIONAL INFO
Alternatively you can perform a standard incision with an automatic device (Surgicutt; ITC, Edison, NJ) and every 15 secs apply a filter paper (Whatman 3MM) for 5 sec until the blood no longer stained it.

REFERENCES
Consequences of GATA-1 deficiency in megakaryocytes and platelets. Carl W; Jackson et al. Blood Vol.93 No9 May 1) 1999 : pp2867-2875