In glow discharge mass spectrometry (GDMS) and glow discharge optical emission spectrometry (GDOES), variations in the signal intensities against the discharge parameters: current, voltage and the pressure of plasma gas, are compared to clarify their excitation/ionization processes responsible to each signal. The behaviors are dependent on the discharge mode, i.e., constant-voltage and constant-pressure conditions. Under the constant-voltage conditions where the gas pressure is varied, the ion intensities in GDMS have a maximum and then decrease at higher discharge currents, while the emission intensities in GDOES monotonically increase along with the discharge current. On the other hand, similar dependence of their signal intensities on the discharge current is observed under the constant-pressure conditions. These results indicate that the transport process of analyte ions produced in the plasma play an important role in determining the ion intensities in GDMS.