Climbing fiber deafferentation of the weanling rat was performed initially by means of intraperitoneal administration of 3-acetylpyridine (3-AP, 65 mg/kg). This dosage of 3-AP, which has been used routinely to achieve olivary ablations in the adult rat, is inappropriate for the total destruction of the inferior olivary complex in weanling rats (21-23 days). Examination of the inferior olivary complex at both the light and electron microscopic levels has revealed a consistent sparing of neurons within the nucleus beta, the genu of the principal nucleus and the caudal portion of the medial accessory olive following long-term (120 days) 3-AP treatment. Complete olivary ablations in weanling rats were obtained only after the injection of a higher dose of 3-AP (90 mg/kg). It would appear that immature inferior olivary neurons are less sensitive to the toxic effect of 3-AP but become increasingly susceptible to 3-AP with maturation. If the appropriate dosage of 3-AP is employed, however, it can be used reliably to produce a chemical ablation of the total climbing fiber input to the cerebellar cortex in weanling rats.