This study investigated factors affecting the metabolic rate of Cancer pagurus eggs. The oxygen uptake of portions of the egg mass and recently hatched zoeae was measured. Oxygen uptake of the eggs of an individual animal was found to vary widely, with measured mean values under normoxic conditions ranging from 38 to 250 μmoles O2 kg−1 min−1. Mean values for the oxygen uptake of zoeae ranged from 958 to 1168 μmoles O2 kg−1 min−1. Mean rates of oxygen uptake of the eggs generally increased with increasing seasonal temperature and advancing developmental stage. Rates of oxygen uptake were maintained over a wide range of ambient oxygen partial pressures. As oxygen partial pressure in the seawater surrounding the eggs fell from a critical value (Pc) of between 60 and 90 mm Hg, the oxygen uptake of the eggs also declined. Mean values for ambient oxygen partial pressures within the egg mass in vivo ranged from 34 to 98 mm Hg. Adult female C. pagurus appear to be able to detect conditions within the egg mass and adjust their egg ventilation behaviour accordingly. Active egg mass ventilation by the adult increased as the eggs became more developed and their oxygen demands increased.