1. The transition state of the glycolytic pathway, under FDP saturating conditions, from no ADP to ADP-saturating levels, is studied in a metabolic model in vitro obtained from rat skeletal muscle. 2. When ADP is absent from the reaction mixture a steady state for NADH concentration is observed. After ADP addition, a new steady state is reached. The transition state from the first steady state to the second one shows a pulse of NADH. Both the profile and the size of this pulse depend on the enzyme concentration. 3. A kinetic model of the lower part of glycolysis (after PFK reaction) is proposed, and this is described by a set of first order coupled nonlinear differential equations. The results obtained through stability analysis and numerical integration of these equations agree with the experimental ones. 4. The possible role of the above mentioned transition state on the transmitter mechanism of glycolytic oscillations from PFK to the lower part of the glycolysis is discussed.