An experimental study is presented for transient buoyancy-induced natural convection in the Rayleigh number range of 6.9 x 10(7)-4.12 x 10(8) for aspect ratios of A(H) = 3 and A(W) = 1.2 inside a rectangular enclosure with silicone oil as the working medium wherein the two vertical walls were heated on the right wall and cooled on the left wall as t greater than or equal to 0, respectively. The remaining four walls were adiabatic. Time evolution of streak dow patterns was observed and temporal temperature distributions are presented and analyzed. In addition, the present results are compared with previous two-dimensional experimental and three-dimensional numerical results.