Lactating mammary epithelial cells synthesize large quantities of milk proteins, which they secrete vectorially at the apical membrane into the alveolar lumen of the gland. Recent work suggested that mammary protein secretion is not wholly constitutive, but may also occur in part through a regulated secretory pathway. This study used mouse mammary epithelial cells cultured on Engelbreth-Holm-Swarm (EHS) matrix to compare the proportions of basally and apically-directed proteins secreted constitutively or in a regulated manner. On EHS matrix, mammary cells formed mammospheres, multicellular structures enshrouded in matrix material, within which they became polarised, formed tight intercellular junctions, and secreted milk proteins vectorially. Protein secreted basolaterally was collected in culture medium, whereas apically-secreted milk proteins accumulated in a closed lumen within the mammosphere, and were recovered by EGTA treatment of the cells in situ. Protein secretion was measured by following the release of radiolabelled protein after pulse-labelling with [35S]-methionine. Basolateral and apical secretion of [35S]-protein appeared complete within 1 h of pulse-labelling, consistent with immediate secretion through constitutive secretory pathways. However, addition of the calcium ionophore ionomycin induced a second wave of secretion in both directions. Ca(2+)-stimulated secretion occurred within 15 min of ionomycin addition, doubled the extent of basolateral and apical secretion, but did not change the populations of proteins secreted. Ionomycin treatment did not affect mammosphere morphology or mammary cell ultrastructure. The results suggest that lactating mammary epithelial cells secrete proteins apically and basolaterally by two pathways, one a Ca(2+)-independent constitutive pathway, the other a regulated pathway stimulated by elevation of intracellular Ca2+.