The objective of the present study was to examine the effects of dietary Zn deficiency on the ex vivo cytokine production (IL-2, interferon-γ (IFN-γ), IL-6 and IL-10) of isolated thymocytes and splenocytes after mitogenic stimulation with concavalin A and to explore the role of corticosterone in this regulation. Weanling rats were assigned to one of four dietary treatments for 3 weeks: Zn-deficient ( < 1 mg Zn/kg diet, ad libitum), pair-fed (30 mg Zn/kg diet, limited to amount of feed as consumed by the Zn-deficient group), marginally Zn-deficient (10 mg Zn/kg diet, ad libitum) and control (30 mg Zn/kg diet, ad libitum). Thymocytes and splenocytes were isolated for cytokine stimulation and determination of T-cell phenotypes. Serum corticosterone concentrations were determined by ELISA. The Zn-deficient and pair-fed groups had 14-fold higher serum corticosterone concentrations compared with the marginally Zn-deficient and control groups (P < 0·0001). The proportions of thymocyte subsets were not altered in the Zn-deficient, pair-fed or marginally Zn-deficient groups; however, thymocyte IL-2 and IL-6 production in these groups was 33–54 % lower compared with the control group (P < 0·05). The Zn-deficient group had an 18–28 % lower proportion of new T-cells (TCRαβ+CD90+), but no difference in the proportion of new T-cells that were cytotoxic or helper. The Zn-deficient group had a 49–62 % lower production of Th1 cytokines (IL-2), but no difference in the production of Th2 cytokines (IL-6, IL-10) by stimulated splenocytes compared with the pair-fed, marginally Zn-deficient and control groups (P < 0·01). These results indicate that Zn status is associated with altered cytokine production, while in vivo corticosterone concentrations are not associated with ex vivo cytokine production.