This preliminary study explores the use of Heart Rate Variability (HRV) as an indicator of user state. In the study, a visual display is used to vary the levels of a complexity factor to assess the impact on user mental effort in a monitoring task Mental effort is measured both subjectively and physiologically. Two findings indicate the potential value of HRV. First, user ERV showed significant discriminatory sensitivity to the manipulation of the independent variable on the basis of domain experience. Second, HRV appeared to indicate the point at which user capacity to process targets was exceeded. Results warrant further investigation but suggest the use of HRV as a tool for design and analysis of user interfaces, and as a research basis for use as component of an adaptive interface.