Trimethylamine (TMA) concentration in fish tissue is an accepted measure of deterioration. In this preliminary study a direct relation was demonstrated between TMA in the tissue and in the headspace above the fish flesh. A relatively simple but precise procedure was developed for measuring TMA headspace concentration, based on air sampling with Carbotrap or Tenax sorbent tubes and subsequent thermal desorption and gas chromatographic (GC) analysis. Sampling and analysis are completed in less than 5 min. Fish freshness determined by TMA headspace analysis was consistent with grading estimated by sensory tests, but the instrumental method provided a more objective and quantitative assessment of quality and an estimation of postmortem age of the fish.