Medicinal plants are valued for their unique complement of bioactive molecules. However in most medicinal plant species, the bioactive compounds have not been completely characterized and the use of these plants as phytopharmaceuticals is based largely on anecdotal evidence. The principle finding of this study was the discovery of melatonin, a neural hormone secreted by the pineal gland in mammals, in three medicinal plants viz. feverfew, Huang-qin and St. John's wort. 'In vitro' regeneration protocols were developed to minimize the potential for the contamination of the plants with indoleamines produced by fungi or bacteria. Plantlets formed via 'de novo' shoot organogenesis induced on intact seedlings, petiole or etiolated hypocotyl explants in response to thidiazuron supplementation of the culture medium. Histological examination of the cultures revealed that the regenerants were primarily 'de novo' developed shoots in all three species. The most prolific regeneration was observed in cultures of St. John's wort and this species was selected for further analyses. For biochemical characterizations, 2-month-old sterile plantlets were excised from the cultures and exposed to 14C-tryptophan in liquid medium. Radiolabel from 14C-tryptophan was recovered as 14C-indoleacetic acid, 14C-hydroxytryptophan, 14C-serotonin and 14C-melatonin. These data suggested several new possibilities for investigation of the metabolism and function of serotonin and melatonin in plant tissues. These possibilities were investigated with inhibitors of auxin and indoleamine metabolism, viz. 2,3,5-triiodobenzoic acid (TIBA), p-chlorophenoxyisobutyric acid (PCIB), p-chlorophenylalanine (p-CPA), d-amphetamine, fluoxetine (Prozac) and methylphenidate (Ritalin). ' In vitro' morphogenesis was found to be significantly affected by relative ratio of serotonin to melatonin thereby poviding the first indication of a role for indoleamine hormones in the regulation of plant morphogenesis.