Abstract Several N-alkanoyl derivatives (4-9 and 13-16) of the potent mitochondrial complex I inhibitor TaClo (1-trichlorom ethyl-1,2,3,4-tetrahydro-β-carboline, 2) have been synthesized in order to elucidate the role of hydrophobic portion in the inhibitory action. Using rat brain homogenates or submitochondrial particles, the inhibitory effects of these compounds towards NADH-ubiquinone reductase (complex I) activity indeed appeared to correlate quite strongly with their lipophilic character. An X-ray structure analysis, exemplarily performed for N-acetyl-TaClo (4), revealed the N-substituent of such chlorinated agents to be dramatically pushed out of the β-carboline ring ‘plane’ due to the high steric demand of the huge trichloromethyl group at C-1.