Alaproclate (2-/4-chlorophenyl/-1,1-dimethyl-2-amino-propanoate hydrochloride), a new selective 5-HT uptake inhibitor has been studied for its in vivo effects on the 5-HT transmission in rats in the following tests: 1) the flexor reflex in the spinal rat, 2) hyperthermia in rats at a high ambient temperature, 3) rat stomach fundus strip preparation. Alaproclate prevented potentiation of the flexor reflex in spinal rats induced by 5-HT releasers fenfluramine and p-chloroamphetamine (PCA) but did not affect the potentiation of the reflex due to direct 5-HT agonists LSD and quipazine. Also hyperthermic response of rats to fenfluramine was antagonized by alaproclate, whereas that due to quipazine was potentiated. Alaproclate failed to inhibit 5-HT-induced contractile response in rat stomach fundus strip preparation. The results confirm in vivo 5-HT uptake inhibition by alaproclate in the central nervous system, and its lack of blocking effect on postsynaptic 5-HT receptors.