Metallic nanoparticles, due to their reduced size and high surface area, have unique properties that facilitate their application, with excellent results, in various scientific fields and allows an easy functionalization of their surface with a variety of ligands. Noble metallic nanoparticles play a special role in nanomedicine due to their proved antibacterial, antifungal and strong antioxidant activity. These metallic nanoparticles can be prepared using both conventional and unconventional routes and, in the last decades, the methods involving plants and their aqueous extracts are constantly and successfully replacing expensive and time - consuming conventional chemical methods. This paper describes the one - pot green synthesis of silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) from aqueous plant extracts and corresponding metallic salts under different time and temperature conditions. Both silver and gold nanoparticles are green synthesized from plants with important pharmacological benefits to human health: Hyssopus (Hyssopus officinalis), European goldenrod (Solidago virgaurea) and Knotgrass (Polygonum aviculare) and their synthesis is monitored by recording the UV - Vis spectra at specific time intervals: 0s, 5 min, 15 min, 30 min, 1 hour and 24 hours in order to evaluate their stability in time. FTIR spectra were used for the specific determination of functional groups and DLS was used to determine the nanometer scale of the particles. Also, antioxidant activity was evaluated using the DPPH method for both silver and gold nanoparticles and compared to that of the crude aqueous extracts.