Many nuclear reactors are constructed of arrays of parallel channels. In order to carry out heat-transfer and flow-redistribution calculations for such arrays, proper boundary conditions must be assigned. These boundary conditions are not always obvious. In this paper, a method of formulating boundary conditions is discussed in which the stagnation streamline is used to define fictitious channel extensions upstream and downstream of the original parallel-channel configuration. This procedure is equivalent to defining a new parallel-channel configuration for which the boundary conditions are more clearly defined. A comparison of the calculated hydraulic parameters with the associated experimental results shows that the method is essentially correct.