LIFETIME MEASUREMENTS OF RYDBERG STATES USING A SAMPLE OF COLD ATOMS A. L. Oliveira, R. A.D.S. Zanon, K. M. F. Magalh\~aes and L.G. Marcassa Instituto de F\'{\i}sica de S\~ao Carlos Universidade de S\~ao Paulo Cx. Postal 369 - S\~ao Carlos -SP - Brasil 13.560 - 970 We have carried out lifetime measurements of Rydberg states in a sample of cold trapped Rb. The Rydberg atoms were detected by pulsed field ionization technique. Due to the low velocity of the atoms we can observe the time evolution of the Rydberg population during very long periods ($\gt 30 \mu $s) for high n (n$\gt 25$). Collisional effects are also observed.