The biosynthesis of mesembrine and related alkaloids occurring in Scelletium strictum L. Bol. has been investigated by administering L-[S-methyl-¹⁴C]methionine, DL-[2-¹⁴C]tyrosine, DL-[3-¹⁴C]tyrosine, DL-[ring-¹⁴C]phenylalanine, and DL-[3-¹⁴C]phenylalanine into the stems of live plants of this species. Chemical degradations of labeled alkaloids obtained in these experiments are used to establish that the aromatic ring in the mesembrine alkaloids is derived from the aromatic ring of phenylalanine but not tyrosine and that the perhydroindole moiety is derived from tyrosine and not phenylalanine. It was found that the S-methyl group of L-methionine provides the O- and N-methyl groups in these alkaloids.