Steroidal glycoalkaloid aglycones (SGAA) in the somatic hybrids between the cultivated potato (Solanum tuberosum line PDH40) and the non-tuber- bearing wild potato species S. brevidens (accession CPC 2451) were analysed as trimethylsilyl derivatives by gas chromatography-mass spectrometry (GC- MS). The reproducibility of the quantitative results including extraction, hydrolysis, derivatisation of aglycones, and GC-MS ranged 5-7% (C.V.). The leaves of S. tuberosum contained solanidine (396 mg/kg dry weight) and solanthrene (49 mg/kg dry weight), whereas the leaves of S. brevidens contained tomatidine (8173 mg/kg dry weight). In addition to these parental- type SGAAs, all somatic hybrids contained demissidine in the leaves, and the total SGAA contents ranged 290-7774 mg/kg dry weight. The pattern of SGAAs produced in the tubers of S. tuberosum and a few tested hybrid lines was similar to the leaves. In the symmetric somatic hybrids which had a known genome composition the proportion of tomatidine of the total SGAA content correlated positively with the genome doses of S. brevidens. Based on the results, a simple hypothesis for the formation of the novel SGAA, demissidine, in the somatic hybrids is proposed. According to this hypothesis, the hydrogenase enzyme of S. brevidens which produces tomatidine from its precursor teinemine by hydrogenating the double bond at position 5 also produces demissidine by hydrogenating the corresponding double bond in solanidine.