OBJECTIVES: To determine (1) whether heart rate variability (HRV) was a sensitive and reliable measure in mental effort tasks carried out by healthy seniors and (2) whether non-linear approaches to HRV analysis, in addition to traditional time and frequency domain approaches were useful to study such effects. METHODS: Forty healthy seniors performed two visual working memory tasks requiring different levels of mental effort, while ECG was recorded. They underwent the same tasks and recordings 2 weeks later. Traditional and 13 non-linear indices of HRV including Poincare, entropy and detrended fluctuation analysis (DFA) were determined. RESULTS: Time domain, especially mean R-R interval (RRI), frequency domain and, among non-linear parameters - Poincare and DFA were the most reliable indices. Mean RRI, time domain and Poincare were also the most sensitive to different mental effort task loads and had the largest effect size. CONCLUSIONS: Overall, linear measures were the most sensitive and reliable indices to mental effort. In non-linear measures, Poincare was the most reliable and sensitive, suggesting possible usefulness as an independent marker in cognitive function tasks in healthy seniors. SIGNIFICANCE: A large number of HRV parameters was both reliable as well as sensitive indices of mental effort, although the simple linear methods were the most sensitive.