Summary 1 . Current studies indicate that estimates of thermal tolerance limits in ectotherms depend on the experimental protocol used, with slower and presumably more ecologically relevant rates of warming negatively affecting the upper thermal limits (CT max ). Recent empirical evidence also gives credence to earlier speculations suggesting that estimates of heritability could drop with slower heating rates. 2 . Using published data from the fruit fly Drosophila melanogaster , we show that empirical patterns can be explained if flies’ physical condition decreases with experimental time in thermal tolerance assays. This problem could even overshadow potential benefits of thermal acclimation, also suggesting that a drop in CT max with slower heating rates does not necessarily rule out beneficial acclimatory responses. 3 . Numerical results from a simple illustrative model show that no clear conclusions can be obtained on how the phenotypic variance in CT max will be affected with different rates of thermal change. Conversely, the genetic variance and estimated heritabilities are expected to drop with slower heating rates, hence ramping rates in experiments aiming to study the evolutionary potential of thermal tolerance to respond to global warming should be as fast as possible (within the range in which measurement accuracy and physical condition are not affected). 4 . Measurements under ecologically realistic warming rates should also consider the impact of other physiological and behavioural strategies that might partly compensate the negative effects of slow heating rates. However, there are situations in which slow heating rates closely mimic natural conditions, as those encountered by some aquatic ectotherms. These heating rates may be an issue of major concern in these species, given its negative impact on CT max and its adaptive potential.