We provide evidence that normal plasma zinc levels in snakes are in the same range as the elevated zinc levels associated with haemolytic anaemia and fatal zinc toxicosis in dogs, and with weight loss and anorexia in crocodiles that had ingested coins with high zinc content. Blood plasma samples from large representative groups of snakes, lizards, turtles and alligators were analysed for zinc content by atomic absorption spectrophotometry. Plasma zinc levels in all snake species were five to 50-fold greater than levels reported in mammals. Plasma zinc levels in lizards and turtles were also higher than those of mammals, but significantly lower than those of snakes. Plasma zinc levels in alligators were in the same range as mammals and birds. After prolonged dialysis of snake plasma, 76% of the zinc remains in the retentate, suggesting the presence of a plasma protein with a strong affinity for the metal. Zinc levels vary significantly among the taxonomic groups, emphasizing the common evolutionary origin of crocodiles and birds and their divergence from the other reptiles.