Ring-chlorinated thienylisopropylamines, thiophene analogues of chloroamphetamines, have been synthesized and their effects on serotonergic mechanisms in the rat brain have been evaluated. With 4,5-dichlorothienylisopropylamine (3e), a pharmacological profile similar to that of p-chloroamphetamine, consisting in a marked and long-lasting serotonin depletion and a rather strong and prolonged inhibition of synaptosomal uptake of serotonin, was found. Chloro substitution in position C3 of the thiophene ring did not determine brain serotonin depletion nor serotonin uptake inhibition but enhanced brain MAO inhibitory activity present in all these compounds. 3,5-Dichlorothienylisopropylamine (3g) was the only compound of the series in which the inhibition of serotonin uptake was more marked than the serotonin depleting property.