Although blood pressure can easily be measured in anesthetized rats by simply connecting a catheter to a pressure transducer, repeated measurements taken over long periods of time in awake rats are much more difficult to make. For chronic experiments two methods are now commonly used: direct recording from chronically-implanted arterial catheters, or indirect measurement with the tail-cuff method. Direct recording of intraarterial pressure can be done continuously and is more accurate, but technically more demanding. On the other hand, although tail-cuff measurements are less accurate, they do not require surgery and can be repeated almost indefinitely. With most tail-cuff methods the rats are preheated to dilate the tail vessels and thereby facilitate pulse detection, but with the new IITC photoelectric sensor indirect measurements of systolic as well as of mean arterial pressure can be made without external preheating. Even with a properly validated tail-cuff method, however, errors can still occur particularly when it is used to quantify modest blood pressure changes like those during development of hypertension, or following administration of vasoactive drugs. To safeguard against such errors, each laboratory should always validate its own tail-cuff method under uniform experimental conditions similar to those existing when the method is actually used. Additionally, all blood pressure differences thereby detected should be verified by direct measurement of intraarterial pressure in the same rats.