This report discusses the experience of a two-year family planning and maternal/child health project in Nepal. Although the project was planned as an experimental field research endeavor, a series of unanticipated events repeatedly compromised the internal validity of the project and forced design changes. While unexpected events are common in the history of most field projects, they present the research evaluator with the fundamental dilemma of trying to maintain a high degree of internal validity without sacrificing external validity. Rigid research designs with tight control over the introduction and measurement of experimental variables may serve to increase internal validity but they may also create an atypical and artificial situation that fails to mirror real field conditions and thus threatens external validity.