The amount and distribution of sulphur in the zona pellucida from mouse, hamster and rabbit oocytes was investigated by means of X-ray microanalysis. The amount of sulphur ranges from 2853 to 10178 mmol/Kg dry weight. The highest amount of sulphur was found in hamster oocytes and the lowest in rabbit oocytes, being the amount found in mouse oocytes intermediate between both values. A gradual decrease in sulphur content from the inner region towards the outer region of the zona pellucida was detected in mouse and rabbit oocytes. This is in agreement with the morphologic and/or biochemical asymmetry described in the oocytes of these species. Although a biochemical and functional asymmetry has been described in the zona pellucida of hamster oocytes, no asymmetric distribution of sulphur has been detected in the present analysis. However, a stratification of sulphur in the zona pellucida from hamster oocytes is evident. The present results suggest that there is a relationship between the amount of sulphur in the zona pellucida and the intensity of zona reaction in mammalian oocytes