beta-Pyrazol-1-yl-DL-alanine, an uncommon amino acid from plants of the Cucurbitaceae, was fed to mice. Although pyrazole is known to affect the liver enzymes UDP-glucose dehydrogenase, UDP-glucuronyl transferase and UDP-glucuronic acid pyrophosphatase, and also depresses their liver glycogen concentrations, beta-pyrazol-1-ylalanine had no such effects. beta-Pyrazol-1-ylalanine could not be detected in the liver of the experimental animals but was present in the urine. No other change in urinary amino acid content was observed. Studies with [14C]-beta-pyrazol-1-yl-DL-alanine showed the administered amino acid was excreted over a 4-day period, 93% of the compound supplied was recovered. Similar recoveries were obtained with the L-enantiomer from cucumber seed. The metabolic inertness of beta-pyrazol-1-ylalanine was also apparent in experiments involving subcutaneous injection of this compound. Administration of pyrazole confirmed an earlier report of resultant increased activity of liver UDP-glucose dehydrogenase and UDP-glucuronyl transferase, and of the depression of activity of liver UDP-glucuronic acid pyrophosphatase. A concomitant 40% decrease in liver glycogen content was seen. The urine contained a novel metabolite, identified as a peptide conjugate of a pyrazole derivative. Mass spectrometry and p.m.r. spectroscopy indicate that this derivative is 3,4,4-trimethyl-5-pyrazolone. The amino acid constituents are aspartic acid, threonine, serine, glutamic acid, proline, glycine, alanine, valine and leucine. The urine of mice receiving pyrazole contained less free glycine and alanine than controls. From the results, it is concluded that pyrazole is not a catabolite of dietary beta-pyrazol-1-ylalanine but to the contrary, the amino acid is essentially excreted unchanged. Formation of 3,4,4-trimethyl-5-pyrazolone from pyrazole would imply C-methylation, a process that has not been previously observed in a mammalian detoxication context.