The purpose of the present study is to show that dietary caffeine, heart tissue Zn levels, and metallothionein (MT) concentration are all interrelated in newborn rats. Dams with eight pups in the control group were fed a 20% protein diet upon delivery, whereas dams in the experimental group were fed a 20% protein diet supplemented with caffeine (2 mg/100 g body weight). Offspring were killed at d 22 postbirth and the hearts and livers were removed to determine the Zn and MT concentrations. Hearts of the newborn rats in the caffeine group showed decreased Zn levels as well as decreased MT concentration. To explain the observed effects in newborn rats, the relationship between Zn and MT levels was studied in adult female rats. They were injected intraperitoneally with either ZnCl2(20 mg/kg body weight) dissolved in saline solution or ZnCl2and caffeine (4 mg/kg body weight) over a period of 2 d. Injection of ZnCl2into adult female rats resulted in an increase in heart MT levels, whereas injection of caffeine caused decreased Zn levels and MT concentration. Current findings indicate that dietary caffeine intake during the lactational period by newborn pups causes a decrease of the heart Zn and MT levels. (Pediatr Res 32: 330–332, 1992)