Notwithstanding the wide range of biological and pharmacological activities reported for sweet basil (Ocimum basilicum L.), many discrepancies are still present in the evaluation of its health-promoting properties. These discordances could be at least in part due to insufficient details of qualitative and quantitative composition, connected to the ample variability of this species. Furthermore, many investigations have been carried out in vitro, with few data available on the effectiveness in biological systems. In this study, the protective effect of essential oils and water-soluble extracts derived from three different cultivars of sweet basil has been evaluated in cultured cardiomyocytes. To verify the effectiveness of supplemented oils/extracts in counteracting oxidative damage, cardiomyocytes were stressed by the addition of hydrogen peroxide. The results indicate that (a) in vitro antioxidant activity is not predictive of biological activity and (b) basil can yield extracts with substantially different protective effects, in relation to composition and extraction techniques. Variation among different cultivars has also been detected.