The escape probability method for treating the transfer of line radiation in a stellar envelope which is in rapid radial expansion is developed, including the effect of radiation from the stellar core. This method is applied to a line formed by a two-level atom, yielding an explicit expression for the line source function. These results are used to calculate three representative line profiles assuming different distributions of absorbing atom density but with a fixed velocity distribution. It is not possible to infer unique values of the parameters from observed line profiles; however, the following qualitative conclusions can be drawn. Flat-topped line profiles, with or without violet absorption, are most naturally produced by an optically thin hollow sphere of emitting material, and rounded lines are the usual result of an optically thick envelope.