Tracer experiments revealed that (10-3H)-loganin (2) was incorporated into gentiopicroside (3) in Gentiana triflora var. japonica. (7-3H)-2 and (carbo-14C-methoxy)-secolo-ganin (10) were also found to be incorporated into morroniside (4) in Gentiana thunbergii and Cornus officinalis, respectively. The biosynthetic pathway of oleuropein-type glucosides in Olea europaea and Jasminum primulinum was further examined by employing (10-3H)-7-deoxyloganic acid (1), (10-3H)-loganin (2), (carbo-14C-methoxy)-secologanin (10), (8-3H)-and (carbo-3H-methoxy)-kingiside (11), together with (8-3H)-and (carbo-14C-methoxy)-8-epikingiside (18).