Abstract – Effects of therapeutic and pulp protecting dental materials on nervous tissue were studied in an in vitro model utilizing the isolated left rat phrenic nerve. The nerve was placed in a buffer bath between two suction electrodes, one for electrical stimulation and one for recording. Evoked compound action potentials (cAP) were monitored in an oscilloscope. Fresh mixes of six commonly used pulp‐dentin materials were placed in contact with the nerve, and the cAP registered. Formocresol, zinc oxide‐eugenol (ZOE), Calasept® and Dycal® caused a complete depression of the nerve activity within 0.5 to 5 min of contact. The depression appeared irreversible with Formocresol and Calasept, reversible with Dycal, and reversible with ZOE when applied to nerves surrounded by adherent fatty tissue. De Trey Zinc® and Durelon® caused only a minor depression of the cAP within 30 min of contact.