A series of 2,6-disubstituted piperidine alkaloids have been isolated from several Pinus (pine) and Picea (spruce) species and characterized structurally. The pines appear to contain only cisdisubstituted piperidines, while the spruces contain both cis- and trans-disubstituted piperidines. The structural relationships among the alkaloids suggest a plausible biosynthetic scheme and a reason why previous attempts to elucidate the biosynthesis of pinidine failed beyond establishing ita polyketide origin. A mixture of alkaloids from needles of Pinus ponderosa proved to be highly teratogenic. The alkaloids might therefore be involved in so-called pine needle abortion which occurs in pregnant range cows which feed on Ponderosa pine needles.