Enhancement and reduction of the fluorescence intensity in Rb atomic vapor are observed. The reduction is due to a depopulation of the 5P - {3/2 } state upon illumination of a Rb vapor cell with light corresponding to the 5P - {3/2 }→5D - {5/2 } transition. Collisional transfer of the 5D - {5/2 } atoms to the 5D - {3/2 } state

followed by radiative decay to the 5P - {1/2 } state

creates an increase in the population of the 5P - {1/2 } state. The increase in the excited state population results in an increase in the fluorescence intensity of the 5P - {1/2 }→5S - {1/2 } transition. The laser-induced population changes are monitored by the decrease or increase of the corresponding fluorescence signals. Using the steady-state rate equations

the rate constants for the stimulated emission(i.e.

5D - {5/2 }→5P - {3/2 }) is 2.0×10 +8s + {-1 }. The cross section for the fine-structure changing collision(i.e.

5D - {5/2 }→5D - {3/2 }) is 6.4×10 + {-14 }cm +2.