From the rhizomes of Petasites hybridus (L.) GAERTN., MAYER et SCHERB., a substance of composition C15H23NO was isolated, which was shown to be the first eremophilane type alkaloid, eremophilene lactam (I). A not yet closely studied mixture containing substances of the same type, with compositions C15H23NO2 (III) and C15H23NO3 (IV), was isolated from the mother liquors. Although Senecioneae tribe plants are known to contain alkaloids, nitrogen-containing compounds were isolated from the Petasites genus only recently; Japanese authors isolated two pyrrolizidine type alkaloids, petasitenine and its acetyl derivative neopetasitenine, from P. japonicus (SIEB. et ZUCC.) MAXIM. On repeated isolation of eremophilane type substances from P. hybridus, a nitrogen-containing analogue of eremophilenolide was isolated and its structure determined. Substance I was isolated from polar fractions of a light petroleum extract of the plant via silica gel chromatography. Its composition C15H23NO was confirmed by mass spectrometry. Its UV spectrum, displaying λmax at 221, indicates the presence of a conjugated carbonyl group. From its IR spectrum (frequencies at 3464 cm-1 for an -NH-group and 3220 cm-1 for the associated form, and bands at 1703.2 cm-1 and 1688.4 cm-1) it follows that it is a lactam group. The 1H-NMR spectrum (100 MHz) showed characteristic signals typical of substances of eremophilenolide type, hence it could be assumed that compound I also probably had a structure of this type, i.e., eremophilene lactam. This assumption was confirmed by comparison of the characteristic signals in the 1H-NMR spectra of eremophilene lactam I and eremophilenolide II (Table I). The sole important difference was in the chemical shift of the signal H(8), caused by the primary different electronegativity of the lactam and lactone groups. Further crystalline fractions were obtained from the mother liquors after crystallization of eremophilene lactam. Mass spectral measurements indicated they were mixtures of substances C15H23NO2 (III) and C15H23NO3 (IV), with compound IV prevailing. The IR spectrum had an absorption maximum at carbonyl group frequency (1707 cm-1) but no band for the hydroxyl group.