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Piper capense L.f. (Piperaceae) is used traditionally in South Africa as a sleep inducing remedy. Bioassay-guided fractionation of the roots of P. capense led to the isolation of piperine (1) and 4,5-dihydropiperine (2), which showed moderate affinity for the benzodiazepine site on the GABAA receptor (IC50 values of 1.2 mM and 1.0 mM, respectively). The present study suggests that strict structural properties of the amides are essential for affinity. Taken together, these observations suggest that the carbon chain must contain not less than four carbons, and that a conjugated double bond, adjacent to the amide group, is necessary for binding to the receptor and that the amine part should be bulky.