We describe a method and its implementation for self-monitoring during natural language generation. In situations of communication where the generation of ambiguous utterances should be avoided our method is able to compute an unambiguous utterance for a given semantic input. The proposed method is based on a very strict integration of parsing and generation. During the monitored generation step, a previously generated (possibly) ambiguous utterance is parsed and the obtained alternative derivation trees are used as a 'guide' for re-generating the utterance. To achieve such an integrated approach the underlying grammar must be reversible.