An imporved method is described for measuring low levels of prostaglandin F2alpha in small volumes of cerebrospinal fluid. Single ion monitoring of the ion at m/e 423, derived from the methyl ester tri-TMS ether derivatives of prostaglandin F2alpha and the internal standard, w-trinor-16-cyclohexyl prostaglandin F2alpha, is employed. Injection of 2 pg of prostaglandin F2 alpha derivative gave a standard deviation of +/- 9%. The majority of patients had cerebrospinal fluid levels of up to 2 ng ml-1 of prostaglandin F2 alpha, whereas in certain neurological conditions with progressive disease, elevated levels were encountered, some of which were higher than 10 ng ml-1.