ABSTRACT. To establish a parameter for zinc status that is independent of the occurrence of infection, we studied the effects of low dietary zinc and endotoxin in weaning rats 21 d after 65Zn intubation. We monitored aspects of zinc status (tissue zinc content, 65Zn distribution, and specific 6sZn activity in tissue) and 65Zn metabolism (absorption, excretion, and biologic half-life), as well as weight gain, feed conversion, and dietary zinc use. The low zinc diet induced classical deficiency with losses of bone zinc, resulting in lower content (7.4 versus 19.6 pmol) and higher spec act (17 versus 8 kBqlpmo1). Other tissue-specific and plasma-specific activities were also higher (overall, 20 versus 8 kBq/pmol; plasma, 8 versus 4 kBq/pmol). Endotoxin caused lower total-plasma zinc (0.04 versus 0.05 pmol) but did not affect spec act (4 kBq/pmol); combined endotoxin and low-zinc diet caused low total-plasma zinc (0.01 pmol) and high spec act, as did the low-zinc diet alone (12 kBq/pmol). We conclude that plasma-spec act (or stable isotope enrichment) can serve as an index for nutritional zinc status during recurrent infection. (Pediatr Res 28: 332-335,1990) Groups of children suffering from recurrent upper respiratory tract infection were found to have low hair zinc (1) and high urinary zinc values, as well as lower SD scores of height for age (2). However, these parameters are considered poor indicators of the zinc status of individual patients (3). Moreover, the relationship between zinc status and the health of a child is ambiguous: deficiency either predisposes to recurrent infection (4) or is induced by it (5). In this study involving young male rats, we monitored the zinc status as affected by low dietary zinc and repeated endotoxin exposure. Weight gain, efficiency of feed conversion, and dietary zinc use were measured. Zinc status was characterized by tissue zinc contents, tissue 65Zn distribution, and specific 65Zn activities (65Zn activity per mol zinc). Regulation of the zinc status was measured by whole-body 65Zn absorption, excretion, and turnover. On the basis of these results, we evaluated the suitability of the parameters studied to discriminate between low zinc intake, endotoxin exposure, and their combination.