The formalin test was developed using an ordinal scale of weighted scores to rate the intensity of pain-related behaviours in animals. However, no studies have been carried out to establish the ordinal relationship of the behavioural categories used to generate the weighted pain intensity scores. The purpose of the present study was to evaluate the validity of the weighted-scores technique by assessing the ordinality of the behavioural categories associated with the specific category weights. The amount of time spent in each of 4 behavioural categories was measured as a function of the concentration of the formalin solution injected into the hindpaw of rats, and as a function of the dose of systemic morphine given to rats injected with a concentrated (5.0%) solution of formalin. The ordinal nature of the category weights was supported when the data were subjected to a polychotomous logistic regression for fitting an ordinal model.