Gold nanorods in colloidal solution can be melted into spherical nanoparticles by excitation with intense femtosecond laser pulses of the proper energy. The threshold of the laser pulse energy for the complete melting of the nanorods with a mean aspect ratio of 4.1 in solution is determined by observing the change in the absorption intensity of the longitudinal absorption band (measure of the rod concentration) at 800 nm with increasing number of laser pulses of known energy. The number of laser pulses needed to reduce the band intensity (rod concentration) by 1/e of its initial value is determined as the laser energy per pulse increases. For pulses of lower energy than threshold, it is found that the number of pulses required to melt the gold nanorods present in solution increases significantly with decreasing laser pulse energy. Above threshold, this number is constant since the additional absorbed laser energy will only further heat the particles to temperatures above their melting point. The gold concentration in the colloidal solution is measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES), from which the gold nanorod concentration is determined from the known shape and size distribution obtained from transmission electron microscopy (TEM) results. A simple analysis using the determined threshold energy and the nanorod concentration showed that it takes an average of similar to 60 femtojoule (fJ) to melt a single gold nanorod. Experiments using 820 nm as well as 410 nm femtosecond laser pulses yield similar values, indicating that the laser induced shape transformation of the nanorods is independent of the irradiation wavelength and that this process is therefore photothermal in origin. (C) 2001 American Institute of Physics.