The parameters governing the response time of a system to follow transient changes of conductivity caused by short pulses of radiation are discussed, and designs for coaxial conductivity cells with parallel‐plate electrodes to obtain subnanosecond time resolution are described. The performance of the system is evaluated using ionization pulses of 30‐ps width. This yields values in agreement with theory: 95 ps for the rise time and 80 ps for the fall time with virtually no signal distortion. Ways to decrease these times further are discussed.