It is established that lithium is accumulated in the above-ground part of agrimony (Agrimoniapilosa L.), the aqueous extract of which exhibits lithium-specific chronobiological activity in rats. The chronobiological study was carried out during the winter solstice on 40 adult male rats divided into three groups: control (intact animals treated with purified water and the extract purified from lithium) and two test groups. Animals in the test groups were administered the aqueous extract of A. pilosa in the morning (8.00 am) and evening (19.00 pm) in amount corresponding to receiving lithium hydroxybutyrate in a dose of 10 mg/kg (p.o.). From the sixth to seventh day of experiment, without canceling this treatment, pilot testing (open field test, rectal temperature measurement) was started at 9 am and continued every 4 h over a period of 48 hours. The primary chronograms were statistically processing by analysis of variance, spectral analysis, and cosinor analysis. The quantitative determination of lithium in the above-ground part of A. pilosa L., its extract and brain of animals was carried out by flame photometry on a SOLAAR Series S spectrometer (Thermo Scientific Co., United States) equipped with a flaming atomizer operating in increased deuterium background correction emission mode. It was found that aqueous extract of the above-ground part of A. pilosa L. contained up to 8.5 ± 0.4 μg/g lithium; in the extract purified by ion-exchange chromatography, this amount was reduced to 1.4 ± 0.2 pg/g. All control animals exhibited external and internal desynchronism. Under the action of herbal extract, the daily dynamics of behavioral activity and rectal temperature in animals acquired a rhythmic character and became consistent with the external day-night cycle.