The effects of storage under frozen conditions on clenbuterol recovery from spiked calf urine samples were studied. Urine samples contaminated with 10 micro g l(-1) clenbuterol and frozen at -15 degrees C were analysed every 15 days over 6 months. A single frozen contaminated urine sample was also thawed every 15 days over 3 months for the analysis of 10-ml aliquots, after which the remaining portion was frozen again (-15 degrees C). The presence of clenbuterol was determined by GC-MS. A gradual decline in clenbuterol recovery was observed from the first to the 180th day. It was only possible to recover 1.74 +/- 0.06 micro g l(-1) (17%) clenbuterol on the 180th day in samples stored at -15 degrees C. Likewise, clenbuterol totally disappeared from spiked urine samples that had been successively frozen and thawed over 3 months. The results were confirmed in a study performed on 2704 calf urine samples collected on farms and analysed by HPTLC and GC-MS. Of 73 positive samples, 61 had been frozen for <10 days.