Gene-targeting studies of pain, using transgenic 'knock-out' mice possessing null mutations of pain-relevant genes, are becoming increasingly common. This approach is a potentially powerful tool for the molecular dissection of complex traits such as pain modulation, but is subject to several theoretical drawbacks. One problem arises from the fact that the genetic background of knock-out mice is virtually always a mixture of alleles from two different strains; commonly 129 and C57BL/6. A more general caveat to knock-out findings derives from the demonstration that null mutations interact with genetic background to produce phenotypic changes. The present study investigated basal' nociceptive sensitivity (on the 49 degrees C tail-immersion/withdrawal test) and sensitivity to morphine antinociception in 129 and C57BL/6 mice (129/J, 129/Sv-+(p)+(Tyr-c)+(Mgf-SIJ), and C57BL/6J substrains). C57BL/6 mice displayed almost two-fold greater initial sensitivity to thermal stimulation than 129 mice, and three-fold reduced sensitivity to morphine inhibition of that noxious stimulus. These findings suggest that gene targeting studies of pain are particularly subject to the aforementioned concerns, and that C57BL/6 mice represent a suboptimal background strain for such efforts.