Semi-quantitative analysis (e.g. optical density ratios) of [14C]2-deoxyglucose autoradiograms is widely used in neuroscience research. We demonstrate that a fixed ratio of 14C-concentrations in the CNS does not yield a constant optical density ratio but is dependent upon the exposure time in the preparation of the autoradiograms and the absolute amounts of 14C from which the concentration ratio is derived. The failure of a fixed glucose utilization ratio to result in a constant optical density ratio represents a major interpretative difficulty in investigations where only semi-quantitative analysis of [14C]2-deoxyglucose autoradiograms is undertaken.