Compound action potential (CAP) recording is a powerful tool for studying the conduction properties and pharmacology of axons in multi-axonal preparations. The sucrose gap technique improves CAP recording by replacing the extracellular solution between the recording electrodes with a non-conductive sucrose solution to minimize extracellular shunting. The double sucrose gap (DSG), conferring similar advantages at the stimulation site, has been extensively used on guinea pig spinal cord white matter (WM) in vitro. Establishing the DSG methodology for WM preparations from smaller animals such as rats and mice is appealing due to their extensive use in basic and translationally oriented research. Here we describe a versatile modular DSG apparatus with rubber membrane separation of the compartments, suitable for WM strips from rat and mouse spinal cord. The small volumes of compartments (<0.1 ml) and the air-tight design allow perfusion rates of 0.5-1 ml/min with faster refreshment rates compared to commonly used 2-3 ml/min and larger compartments, providing economical usage of expensive pharmacological drugs. Our improved DSG design is particularly efficient for uncovering slower conducting, higher threshold CAP components, as demonstrated by recordings of C-wave (non-myelinated axons) in rat dorsal WM. In myelin-deficient Shiverer mice with genetically dysmyelinated axons, our DSG apparatus recordings revealed a multi-peak C-wave without preceding faster components. The improved stimulation and recording with our DSG apparatus, lowering the range of required stimulus intensities and reducing the artifact interference with recorded CAPs provide for critical technical advantages that allow for more detailed analysis of CAPs in relatively short preparations.