Excitability of intracellularly perfused squid giant axons was maintained in hypotonic solutions (down to 300 mOSM) and in hypertonic solutions (up to about 10 OSM), when osmolalities of internal and external solutions were adjusted to be equal with glycerol, glucose, or sucrose. Molar concentrations of ions were kept constant during one series of experiments. The resting potential and the amplitude of the action potential did not change in both hypotonic and hypertonic solutions. With reduction of osmolality, the duration of action potential decreased and the maximum rate of rise and conduction velocity increased. By raising osmolality, the duration was prolonged and the maximum rate of rise and the conduction velocity decreased. Effects of osmolality change were almost reversible. However, these effects were not directly related to the osmolality change but seemed to be related to the viscosity change of the solutions. When the osmolality of external solution was raised with NaCl (up to 2.6 M NaCl), the overshoot increased in porportion to the logarithm of the NaCl concentration. The slope of increase was about 50 mV/decade. However, the resting potential showed little change. With increase of the NaCl concentration, the duration of the action potential increased.