The alkaloidal distribution in young plants of Mitragyna speciosa has been investigated. The major indole alkaloids, as distinct from those in the mature plants, are those possessing C-3Hp with isocorynantheidine and isopaynantheine, two new alkaloids and mitraciliatine (found for the first time in Mitragyna speciosa) being the dominant ones although speciogynine, a C-3Ha alkaloid occurred in the major quantities. The oxindole alkaloids rhynchociline and ciliaphylline were also found in Mitragyna speciosa for the first time - although not in the leaves, while specionoxeine and isospecionoxeine were found for the first time in this species obtained from Thailand. Following the recent work by ZENK there is a discussion on the possible biogenetic route of the Mitragyna alkaloids and a modification of the previous hypothesis is put forward.