An active fluid reservoir for transmission fluid of an automatic transmission, in a first embodiment, comprehends an elongate reservoir disposed adjacent fluid lines from the automatic transmission to the transmission fluid cooler. The reservoir may be associated with either the supply or return line or include two smaller reservoirs associated with both lines. Thermally actuated valves at each end of the reservoir(s) open to allow fluid flow through the reservoir as fluid temperature increases and a diverter valve in the cooler line(s) closes to divert flow into the reservoir. In a second embodiment, the fluid reservoir comprehends a container, tank or similar storage device in fluid communication with a transmission oil cooler line. Again, the device includes three thermally actuated valves.