An approximate analytical method is presented for the estimation of the loss probability for single and multichannel calls in TST (time-space-time) switching networks. Two extended methods are described. The assumptions for the basic method are that: all occupation patterns of the two internal highways between a certain start and destination highway for a given number of occupied channels have the same probability, and that the state probabilities of the start and destination highway are given by a multidimensional Erlang distribution. Based on this method, the second approximation considers calls that are switched from start to destination highway. In a third method, the dependency of the states of all input and output highways is taken into account. The basic approximation has shown the best agreement with the simulation results. The other methods take into account additional properties of TST switching networks for single and multichannel connections. In order to keep the state space in a reasonable size, additional assumptions had to be used which impair the results significantly compared to the basic approximation method.>