Specimens of mouse mammary glands obtained from animals being in different phases of reproductive cycle were collected. After staining MCN, the total mast cell area (TMC), mean mast cell area (MMC), and lactiferous structure density index (LDI) were examined in sections, using a computer image analysis system. Statistically significant (P<0.05) results were: 1. An increase in MCN observed in Group I (4-5 and 9-10 days of pregnancy), a decrease in MCN observed in Group II (2nd and 10th day of lactation); 2. Changes of TMC fully corresponding to changes of MCN; 3. Increase in MMC observed in Group I at mid (days 9-10) and at the end (days 18-19) of pregnancy, decrease in MMC observed in Group IIB (10th day of lactation); 4. LDI (%) higher at the end of pregnancy (Group IC) and during lactation (groups IIA, IIB), compared with control (23.5 ? 4.12, 37.6 ? 3.24, 71.0 ? 4.33 vs. 3.8 ? 0.39). The observed changes in the number and size of MC strictly correspond to physiological phenomena leading to alternation of the mouse mammary gland functional status by development/involution of the lactiferous structures.