The histology and porphyrin content of the Harderian gland and the serum prolactin levels were examined in male B6C3F1 mice treated with neuroleptic buty-rophenones (timiperone and haloperidol) and treated concurrently with timiperone and 2-bromo- α -ergocryptine (bromocriptine), a potent suppressor of prolactin. Light-microscopically, both timiperone and haloperidol increased the number of accretions of porphyrin pigment within the Harderian gland lumina. Timiperone treatment of mice increased both the porphyrin content of the Harderian gland and the serum prolactin levels. Administration of bromocriptine to timiperone-treated mice distinctly prevented the rise in both tissue porphyrin and serum prolactin levels. In intact mice, bromocriptine also exerted inhibitory effects on both the Harderian gland potphyrin content and the serum prolactin level. Electron microscopic investigation revealed that the cytoplasm of type A cells in the Harderian glands of mice treated with timiperone contained trilaminar profiles similar to those seen in the intraluminar pigment masses. These results indicate that timiperone accelerates porphyrin secretion from the type A cells of the mouse Harderian gland by increasing the serum prolactin levels.