| Literature review was firstly conducted to clarify the dietary factors affecting the safety of foods of animal origin. Two experiments were designed: (1) to study the dynamic of Clenbuterol-HCL (CL) in body of pigs by determining its residues in urine at different times while being continuously added or after cease of adding (Expt.l), and (2) to investigate the occurrence of CL from 1999 to 2002 in City of Ningbo, Zhejiang Province (Expt.2).From large number of literature, it is indicated that animal feeds and forages contain a wide range of contaminants and toxins arising from anthropogenic and natural sources. Certain chemical substances and biological agents incorporated into feed, either intentionally or unintentionally, can result in hazards in food of animal origin, and may enter feed at any stage of production up to the point of feeding. In terms of the sources, the feedborne hazards may be classified into four categories: (a) hazards inherent in feeds such as protease inhibitors and lectins; (b) secondary metabolites such as mycotoxins and nitrite; (c) contaminants including chemicals and infectious agents; and (d) artificially adding substances including micro-minerals and even contraband veterinary medicines and chemicals such as p-agonists. In current Chinese situation, the main factors affecting safety of animal feed include chemical agents, drug and microbeadded or/and produced while processing, among which mycotoxins, contraband veterinary medicines and excessive heavy metals, agricultural chemicals are most serious. In this paper, CL was mainly dealt with in two experiments.Forty eight growing-finishing pigs (hybrid from Duroc x Landrace x Large White) weighing 60 kg were used in Expt. 1 that was divided into two stages. At the first stage, samples of urine were collected at different intervals during 10 days from four groups of pigs that were continuously added CL at levels of 0, 1, 2 or 5 ng/kg diet. And the CL was withdrawn from the diet and Expt. 1 entered into Stage two. The samples of urine were then collected at days 1, 2, 6, 9, 12 and 15. In both stages, samples were collected triplicate from each animal for each time of collection. On the 15th and 25th days post-administration, two pigs were slaughtered from each group and samples of livers were collected. Concentrations of CL were determined in all samples of urine and livers by Enzyme-Linked Immunosorbent Assay (ELISA). Concentrations of CL in urine of pigs reached the peak at about 4 hours after feeding irrespective of the adding level, while the peak values varied for different levels and increased with adding level of HCL. The elimination of CL in urine was slow. Its occurrence in urine lasted for more than 10 d, longer with the increasing addition level. The residual time of CL in livers was longer than in urine, usually longer than 15 days, but its residue was found even after more than 25 days when the pigs were fed on diet with high level of CL.Experiment 2 was carried out to investigate the change in occurrence of CL in City of Ningbo during 1999-2000 depending on year, region, locale and source and parts of pigs and spatial distribution. A total of 2 766 samples including urines, hairs, meat and visceral organs of pigs were randomly collected from pig farms (2 455), fixed slaughterhouse (147), farm markets (134) and feed manufacturers (30) by stages and batches during March 1999 to March 2002 and were detected for the residue of CL by ELISA and GC/MS. The occurrence of CL-positive samples was the highest in 2001 (30.7%) and sharply dropped to 8.0 % in 2002, apparently resulting from the standardization of the relevant laws and codes and attention to safety of feed by farmers.Concentration of CL in the positive samples ranged from 1.0 to 270.3 ppb, and was less than 5 ppb in 83.49% of sample (435/521). There existed differences in occurrence of CL between different regions of Ningbo, the yearly variability was similar to the average. The occurrence of CL was obviously higher in the imported pigs (37.4%) f...
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