A theoretical and experimental study of a natural circulation loop is presented. The analysis is based on a one-dimensional model in which the continuity, momentum, and energy equations are given and solved. Expressions for the steady-state and transient flow rates and temperature distributions are derived. The apparatus consisted of an electrically-heated section and two parallel loops with heat exchangers. Steady-state and transient experiments were conducted to study the effects of core flow resistance, power distribution and upper plenum design. Flow oscillations were observed under certain conditions, which were accompanied by instabilities and flow reversals. Reasonable agreement (±30 percent) is obtained between the analytical and experimental results.