This study investigates the effects of tetrodotoxin, a poison from puffer fish, on sodium and potassium currents in lobster giant axons. Tetrodotoxin at concentrations of $1 \times 10^{-7}$ to $5 \times 10^{-9}$ gm/ml blocked action potentials without affecting resting potentials. The increase in sodium conductance during depolarization was suppressed by tetrodotoxin, while the delayed increase in potassium conductance remained unchanged. The results suggest that tetrodotoxin selectively inhibits the sodium-carrying mechanism while keeping the potassium-carrying mechanism intact. This finding supports the hypothesis that tetrodotoxin blocks action potential production through its selective inhibition of sodium channels.This study investigates the effects of tetrodotoxin, a poison from puffer fish, on sodium and potassium currents in lobster giant axons. Tetrodotoxin at concentrations of $1 \times 10^{-7}$ to $5 \times 10^{-9}$ gm/ml blocked action potentials without affecting resting potentials. The increase in sodium conductance during depolarization was suppressed by tetrodotoxin, while the delayed increase in potassium conductance remained unchanged. The results suggest that tetrodotoxin selectively inhibits the sodium-carrying mechanism while keeping the potassium-carrying mechanism intact. This finding supports the hypothesis that tetrodotoxin blocks action potential production through its selective inhibition of sodium channels.