The solution and solid state NMR spectra of 3,4-methylenedioxy-N- methylamphetamine hydrochloride (MDMA. HCl) and a number of illicitly manufactured tablets containing this material and marketed as 'Ecstasy' have been obtained. We show solid state NMR to be a useful technique for the analysis of the impurities and excipients in 'Ecstasy' tablets and with further development may be used quantitatively for determining the percentage carbon which is MDMA. Excipients detected include lactose, cellulose, stearate salts, sucrose, starch, polyvinylpyrrolidone and sodium croscarmellose. Two samples were found to contain 3,4-methylenedioxy-N- ethylamphetamine (MDEA), rather than MDMA. Some interesting conformational infirmation is also observed. Differences in the chemical shifts of C-8 and C-10 carbons for 3,4-methylenedioxy-N-methylamphetamine hydrochloride in solution and solid state and in different 'Ecstasy' samples are attributed to conformational freezing and hydrogen bonding. In the solid state, carbons 8 and 10 are restricted from free rotation and the methyl groups at carbon 10 and carbon 11 are held only in trans conformation unlike in solution. These results were confirmed by a crystal structure analysis. When excipients capable of hydrogen bonding are physically mixed with MDMA · HCl, the chemical shifts of carbons 8 and 10 in the resulting mixture changes such that they more closely resemble the shifts observed in solution.