I describe a highly sensitive and very specific assay for cyclic AMP and cyclic GMP in body fluids. An adaptation of the method of Harper and Brooker [j. cyclic Nucleotide Res. 1,207 (1975)], its advantage lies in the fact that the simple acetylation of the samples--a procedure developed by these workers--greatly increases specificity and sensitivity beyond those of previous methods and permits the measurement of femtomole quantities of both cyclic nucleotides without prepurification. Because the range of the cyclic AMP assay is 20-200 fmol, and that of cyclic GMP 10-15 fmol, experiments can be performed with use of only microliters of fluid. The method requires no extraction and thus no complicated corrections are necessary for analytical recovery.