This series of experiments was eye-opening for me, suggesting that drug tolerance, in particular morphine tolerance, is a LEARNED response (and hence subject to extinction). Please be aware that some background in classical conditioning is required for interpreting the evidence. Please also note that the colony room---no doubt looking and smelling like a typical animal housing unit---must have been dramatically different from the sterile, perhaps cleaning-product-scented environment of the hot plate room. So we should keep that in mind when interpreting the rats' responses, that they must have taken great notice of the hot plate experience.