Proponents of the use of signal detection theory (SDT) in the assessment of pain modulation have generally looked for changes in d′ to indicate a reduction of sensory function, and a change in criterion to indicate a modification of the subject's response bias or attitudinal predisposition. In the first experiment, both assumptions failed to receive empirical verification. Discrimination d′ was equivalent before and after two strong levels of electrical current were reduced. The criterion parameter appeared to shift in a more conservative direction after the stimulus diminution. These results are used to question the validity of both detection and discrimination indices in the measurement of pain. An alternative means for describing the experimental results revealed a striking adaptation-level effect with implications for the assessment of both experimentally induced and endogenous pain. The outcome of a second experiment reinforced the adaptation-level theory interpretation of the results and provided additional evidence concerning the difficulties in evaluating SDT parameters in studies of potential analgesics.