Chloroquine diverts secretory peptides from the regulated to the constitutive secretory pathway. The exact site and mechanism of the effect are not known. We studied the effect of increasing doses of chloroquine on the morphology of cultured melanotrophs from the rat pituitary. 40 microM chloroquine for 2 h, which perturbs intracellular pH gradients in melanotrophs without affecting secretion, caused swelling of a subpopulation of immature secretory granules. 200 microM chloroquine for 2 h, which diverts secretory peptides from the regulated to the constitutive pathway in the AtT-20 cell line, caused pronounced swelling of immature secretory granules, vacuolization of the trans-Golgi region and the appearance of myeloid bodies and multivesicular bodies in the cytoplasm. Golgi stacks were retained and Golgi cisternae only slightly dilated at both chloroquine concentrations. Mature secretory granules were not affected. Cationized ferritin was internalized and transported to the trans-Golgi region in the presence of 40 microM chloroquine while 200 microM chloroquine arrested internalised ferritin in peripheral multivesicular bodies. The study shows a heterogeneous effect of lower doses of chloroquine on immature secretory granules, providing a tool for studies on the relationships between condensation, acidification and peptide processing during granule formation. Chloroquine of 200 microM caused morphological changes typical for chloroquine toxicity and arrest of endocytic traffic.