Traffic control at an ATM multiplexer is of fundamental importance in B-ISDN. Due to the presence of large propagation delay, effective traffic control tends to use only local information. Leaky bucket has been introduced as a rate control method. The present paper studies the system performance of a leaky bucket controller with the arrivals being Poisson processes or Markov modulated Poisson processes (MMPP). Numerical results indicate that a small to medium sized controller is sufficient to attain the capability of a leaky bucket controller; increasing the size of the controller will not offer much improvement in performance. Controlling MMPP type of arrivals with a leaky bucket device may incur low performance especially when the burst duration is long. The performance can be improved by controlling a group of bursty users instead of a single bursty user.