The inhibitory effects of various analogues of imipramine on [3H]‐5‐hydroxytryptamine (5‐HT) uptake into homogenates of rat hypothalamus were examined. For structures with a three carbon side chain the tertiary amine derivative was more potent than the compound with a secondary amine function. Potency was reduced by increasing or decreasing the length of the three carbon side chain by one carbon atom. Substitution of a methyl group in the α or β position in the side chain reduced potency. Replacement of the dimethylene bridge in imipramine by a sulphur atom or substitution of a C=C double bond for the exocyclic N‐C bond of imipramine both led to a fall in potency. 3‐Chlorimipramine was the most potent inhibitor of [3H]‐5‐hydroxytryptamine uptake of the compounds tested.