A method and system of monitoring fluid flow in a wellbore (16). One or more space-distributed temperature input pulses (6a,b; 6′; 6″) are induced into at least a portion of the fluid in a housing (15; 15a,b; 15′; 15″) in the wellbore, The housing has at least one cavity (14; 14a,b) and one or more flow ports (4; 4′. 5; 5′; 5a′,b′). A temperature response pulse, caused by said temperature input pulses, are sensed at one or more locations (2) downstream of said cavity (14; 14a,b) and a flow rate of said fluid through said cavity (14) is determined. The flow rate may be determined based on one or more characteristics of said temperature response pulse, and the Retention Time Distribution (RTD) of said cavity.