Aquaporin-4 (AQP4) water channel is abundant at the sarcolemma of skeletal muscle and presumably plays a substantial role in adapting the rapid changes in fiber volume as well as osmotic pressure during repeated muscle contractions. In this paper we report on the expression of AQP4 and its correlation to the metabolic and functional development of locomotor muscles during the early stage of postnatal development of mouse. The functional development of hindlimb muscles was analyzed by video recording spontaneous walking of mouse pups in specifically constructed walkway. The expression of AQP4 was examined by using immunofluorescence technique and fiber counting. Metabolic development of the muscles was analyzed by using histochemical staining methods for NADH activity and glycogen phosphorylase activity. According to the results, the number of AQP4 positive fibers increases markedly during the first week after birth. This increase in AQP4 expression is accompanied by rapid functional and metabolic maturation of muscles, such as the gradual decrease in the duration of swing phase and the differentiation of glycolytic activity of the muscle fibers. Importantly, most marked increase in AQP4 expression appeared during the first three postnatal days when also most distinct change in functional and glycolytic potential of muscle fibers took place. However, the general locomotor activity of mouse pups appeared confined until one week after birth. The fiber type dependent expression pattern of AQP4 was evident prior to increased locomotor activity at the second week after birth. These results showed for the first time that the amount of AQP4 increases in locomotor muscles during early postnatal days in mouse. However, based on the analysis of locomotor movements, it was concluded that the enhanced water permeability may not contribute to muscle performance until the second postnatal week.