Transient experiments were performed using two natural convection loops in series. The fluid in both loops was water at a pressure of 1 to 10 atm. Measurements were made of the temperature at key points in both loops over the duration of the tests, which were 4 to 6 h long. By using the assumption that after several hours the loops were operating in a quasi-steady-state condition, estimates could be made concerning the fluid circulation rates and heat transfer rates in various parts of the system. The flow rates were very low and in the laminar flow range. There was essentially no time lag before the start of flow in the second loop. The heat exchanger coupling the two loops was of a design whose performance could not be easily predicted. The measurement of key loop temperature as a function of time provides a simple means of obtaining preliminary predictions in planning extensive experimental test programs for complicated thermal systems.