Coloured molecular probes of diverse molecular structure were used to define the permeability of the zona pellucida of preimplantation mouse embryos. The molecular characteristics examined were: electric charge, the size of conjugated aromatic groups and the hydrophilic-lipophilic character, all of which can be described numerically. It was hoped that definition of the types of compound able to cross the zona pellucida would contribute to identification and understanding of potential maternal--embryonic signals. The staining pattern of 51 dyes in unfertilized and fertilized one-cell eggs was recorded and 16 dyes were subsequently used to test two-, four- and eight-cell embryos further, to establish whether the permeability of the zona pellucida altered with developmental age. Four different staining patterns were observed that directly correlated with the numerical parameters of the probe but were independent of developmental age. The size of the conjugated system and the hydrophilic-lipophilic balance of the probe were important in determining its interaction with the zona pellucida and embryo whereas electric charge appeared to have little influence. The resulting model is useful for predicting likely interactions between chemically defined molecules and biological entities and suggests that most biologically active molecules and metabolites can pass through the zona pellucida unhindered and enter the embryo with ease.