Bruce Petrie, a graduate student of Alexander's, attempted a replication correcting for methods issues in the originals and could not replicate the results. Subsequent researchers have shown little interest in replicating the studies further, citing methodological problems in the originals themselves: a small number of subjects, oral morphine dosing that does not mimic real use and introduces a bitterness confound, inconsistent measurement of morphine consumption between conditions, plus equipment failures, lost data and rat deaths.
What would resolve this A modern, adequately powered replication using a consistent, non-oral dosing method and standardized morphine-consumption measurement across all housing conditions, addressing the equipment and data-loss problems that undermined the original series.
OpenBehavioral pharmacology and addiction researchWikipedia: Rat Park