An in vitro radioligand binding assay at A₁ and A_2A adenosine receptors (ARs) was conducted with a fixed extract combination of valerian (Valeriana officinalis) and hop (Humulus lupulus) (Ze 91019) to investigate a possible mechanism for the pharmacological activity of the extract. Component extracts of valerian and hop were also individually investigated. The fixed combination Ze 91019 as well as the valerian extracts therein exhibited selective affinity to A₁ARs (K_i = 0.15–0.37 mg/ml vs. [³H]CCPA). The same extracts exhibited partial agonist activity at the A₁ adenosine receptor as indicated by a lower degree of stimulation of [³⁵S] GTP γS binding in membrane preparations of CHO-hA₁ cells compared with the full A₁ AR agonist N⁶-cyclopentyladenosine (CPA). In addition, valerian extract inhibited cAMP accumulation in CHO-hA¹ cell membranes. The partial agonistic activity at A₁ARs may play a role in the sleep-inducing effect of the valerian.