The objective of the present study was to examine and to compare the impact of severe protein malnutrition during development, adulthood and ageing on secretory immune expression in the eye, mouth and small intestine. In addition, we sought to determine whether potential abrogation of mucosal immunity by protein deprivation might be reversed by the administration of a balanced diet. Weanling, adult and aged rats were provided isocaloric diets containing 24% (control), 19%, 14%, 10%, 6% and/or 3.2% protein levels for defined periods and various immunological parameters were evaluated before, during and after the dietary regimen. Our results demonstrated the following. (1) Severe protein malnutrition (3.2%) dramatically suppressed the secretory immune system in eyes of weanling rats. After 8 weeks of protein insufficiency, tear IgA concentrations in young rats had undergone a precipitous decrease, such that IgA could not be detected in tears. This response was paralleled by a significant decline in the tear volume, tear secretory component (SC), IgG and total protein content, number of IgA-containing cells in lacrimal tissue, as well as the amounts of SC and/or IgA in saliva, intestinal secretions and serum. In contrast, the immunological effects of protein malnutrition in adult or aged animals varied considerably depending upon the specific mucosal site. (2) The influence of protein deprivation was dose dependent and reversible: maintenance of weanling rats on 10%, 6% or 3.2% protein diets interfered with the establishment of ocular and intestinal mucosal immunity, but later administration of optimal diets to these malnourished animals permitted a rapid immune recovery. (3) The impact of protein malnutrition on tear IgA levels in weanling animals, as shown by pair-feeding experiments, appeared to reflect primarily protein deficiency and not caloric restriction. Overall, these findings show that dietary protein plays a significant, site-specific role in the developmental expression of the secretory immune system.