Conditional self-discrimination is defined as a type of stimulus control in which discriminative stimulus is some aspect of the same individual, subsequently associated with an arbitrary stimulus. This capacity is neither exclusively human nor exclusively verbal. In this study, conditional self-discrimination of subjective day/night was explored in Sprague-Dawley and Wistar rats. For this purpose, one experiment was designed in which discriminative stimulus was the natural internal state of the organism with respect to day and night. It was found that both strains are able to discriminate subjective night but not subjective day. No significant differences were found between the strains. Results are discussed mainly in terms of the nocturnal nature of rats and the difficulty of the task.