A revolution in molecular biological technology has allowed, for the first time, the study of pain at the level of the gene. The molecular genetic technique currently garnering the most interest is the use of transgenic mice that either overexpress, or do not express, presumably pain-related proteins. This paper reviews the findings of investigations in which a transgenic mouse has been assessed for nociceptive or analgesic sensitivity. As of this writing, 25 different kinds of mutant mice--lacking neurotrophins and their receptors, peripheral mediators of nociception and hyperalgesia, opioids and their receptors, non-opioid transmitter receptors, and intracellular molecules participating in signal transduction--have been produced and tested on behavioral assays of nociception. Results of these studies have been variously confirmatory, contradictory and enlightening compared to conventional investigations. The advantages and limitations of this approach to pain research are discussed.