We show in this paper that the cell spacing technique can be used to regulate TCP when the traffic of a TCP connection is carried over an ATM connection. For this purpose, we consider a very simple traffic configuration where two workstations exchange TCP data traffic segmented into ATM cells and transmitted over an ATM link traversing a cell spacer. We precisely analyze how the buffer content of the cell spacer evolves over time. The results obtained via very simple mathematical models are then compared with those of a real TCP data transfer experiment.