In order to evaluate the potential usefulness of the drug as an antidepressant, acute and chronic effects of rolipram, a selective inhibitor of Ca2+- and calmodulin-independent cyclic AMP phosphodiesterase were investigated on muricide in olfactory bulbectomized (OB) rats. Upon single administration to OB rats, rolipram at a dosage of 1 mg/kg body weight suppressed the muricide for 2 hr after its administration. As a consequence of daily administration of rolipram, however, the incidence of muricide at 24 hr after the administration was decreased, and more than 60% of the rats did not exhibit the muricide on the 12th day. After the cessation of the administration, the incidence of the muricide returned to the initial level. The suppression of the muricide was not antagonized by several kinds of neurotransmitter blockers. Administrations of phosphodiesterase inhibitors and dibutyryl cyclic AMP as well as desipramine and clomipramine also suppressed the muricide dose-dependently. Repeated administration of desipramine also gave results similar to those of rolipram: repetition of a short suppression on the muricide was followed by the appearance of a long-lasting suppression. Differently from rolipram and desipramine, dibutyryl cyclic AMP did not cause long-lasting suppression, and even the direct effect (75% suppression) observed 30 min after its administration on the first day disappeared during its repeated administration for 14 days. From these results, rolipram was considered to show an antidepressant effect through the inhibition of Ca2+- and calmodulin-independent cyclic AMP phosphodiesterase.