The respiratory properties of maternal, foetal and juvenile whole blood were studied in the ovoviviparous snake, Agkistrodon piscivorus. Haematocrit, haemoglobin concentration, O2 capacity, Bohr effect and Hill coefficients were not significantly different in foetal and maternal blood and were similar to values previously reported from snakes. A significant difference in foetal-maternal blood P50 (foetal 19.5, maternal 48.8) was found. Nucleoside triphosphate (NTP) levels were lower in foetal than in maternal and juvenile snakes. The foetal-maternal difference in P50 and NTP levels disappeared soon after birth (juvenile P50 45.5). Starch gel electrophoresis revealed no difference in foetal and maternal haemoglobins. We suggest that the foetal-maternal shift in blood oxygen affinity is modulated directly and/or indirectly by NTP levels.