The objective of the research was to study the changes of the Endoplasmic Reticulum (ER) in mammary epithelial cells of rabbits during lactation and regression stages. Our results using conventional transmission electron microscopy has shown that ER is changed during the various mammary gland development stages. We found that the average relative volume of ER achieves 17.7±2.464% of cytoplasm volume in the stage of lactation. During regression ER relative volume decreased to 1.274% of the cytoplasm. This difference compared to lactation is statistically significant (p<0.01). Relative surface of ER during lactation reaches 5,630±1,033 μm2/μm3 of cytoplasm. We found a decrease of relative surface of ER to 0.192±0.069 μm2/μm3 of cytoplasm during regression stage. The average size of ER cisternae and tubules reach 0,0953±0,012 μm at lactation stage. During a stage of regression the average size of ER cisternae and tubules is 0.171±0.154 μm. This difference compared to lactation is not statistically significant.
