Current e-learning standardization initiatives have put much effort into easing interoperability between systems and the reusability of contents. For this to be possible, one of the most relevant areas is the definition of a run-time environment, which allows Learning Management Systems to launch, track and communicate with learning objects. However, when dealing with intelligent content, these environments show important restrictions. In this article, we study these restrictions, comparing several standardized run-time environments with nonstandardized solutions that aim to overcome these constraints. (Contains 7 figures, 1 table and 13 notes.)