Pentylenetetrazol (INN), also known as metrazol, pentetrazol, pentamethylenetetrazol, Cardiazol or PTZ, is a drug used as a circulatory and respiratory stimulant. High doses cause convulsions, as discovered by the Hungarian-American neurologist and psychiatrist Ladislas J. Meduna in 1934. It has been used in convulsive therapy, but was never considered to be effective, and side-effects such as seizures were difficult to avoid. Its approval by the FDA was revoked in 1982.

Pentylenetetrazol is considered a GABA antagonist. The mechanism of the epileptogenic action of pentylenetetrazol at the cellular neuronal level is still unclear. Electrophysiological studies have shown it acts at cell membrane level decreasing the recovery time between action potentials by increasing potassium permeability of the axon. Other studies have implicated an increase in membrane currents of several other ions, such as sodium and calcium, leading to an overall increase in excitability of the neuron membrane.

Pentylenetetrazol has been used experimentally to study seizure phenomenon and to identify pharmaceuticals that may control seizure susceptibility. Pentylenetetrazol is also a prototypical anxiogenic drug and, has been extensively utilized in animal models of anxiety. Pentylenetetrazol produces a reliable discriminative stimulus which is largely mediated by the GABA_A receptor. Several classes of compounds can modulate the pentylenetetrazol discriminative stimulus including 5-HT_1A, 5-HT₃, NMDA, glycine, and L-type calcium channel ligands.