A method of determining arterial input function (AIF) by continuously detecting the (17)O MR signal changes of (17)O-labeled water tracer in the rat carotid artery using a region-defined (REDE) implanted vascular RF coil at 9.4 Tesla is reported. This coil has a compact physical size of 1 mm inner diameter, 3 mm outer diameter and 11 mm in length. It can be readily implanted into the rat neck and wrapped around the rat carotid artery for achieving adequate MR detection sensitivity for determining AIF with minimal surgical trauma. Water phantom and in vivo MR experiments were conducted for validating the coil's performance. A signal-to-noise ratio of approximately 20:1 was achieved for the (17)O signal acquired from naturally abundant H(2)(17)O in a small amount of blood (approximately 7 microl) inside the rat carotid artery with an acquisition time of 11 s. The REDE RF coil design electromagnetically isolates the rat carotid artery from surrounding tissues and ensures that the MR signal detected by the RF coil is only attributable to the artery blood. It also minimizes the electromagnetic coupling between the implanted RF coil and a head surface coil tuned at the same operating frequency (two-coil configuration). This configuration allowed simultaneous measurements of dynamic changes of (17)O MR signal of the H(2)(17)O tracer in both rat carotid artery and brain. Compared to most contemporary MR approaches, the REDE implanted RF provides a simple, accurate, and promising solution for determination of AIF in small experimental animals.