The aim of the present study was to determine both the minimal and the optimal conditions under which heat treatment is effective for enhancing 3D (three-dimensional)-like cell proliferation. C3H I 0T 1/2 mouse fibroblasts were cultured with hydroxyapatite granules for 10 weeks after heat treatment at 40, 41.5, 43, 44 or 45 degrees C for 2, 10, IS, 20, 30, 45, 60, 90, 180 and 360 min. Then minimal and optimal conditions of temperature and duration of heat treatment for induction of 3D-like proliferation of cells were determined. The minimal conditions of heat treatment to induce 3D-like proliferation were 43 degrees C for 2 min and the optimal conditions were 43 degrees C for 10 min. The mean rates of formation of 3D-like proliferation patterns by cells heat-treated at 43 degrees C for 2 and 10 min were significantly higher (1.7- and 3.7-fold respectively) than that by untreated cells (P < 0.05). We also observed significantly greater effects of heat treatment on 3D-like proliferation at 40 degrees C for 90 or 180 min and at 41.5 degrees C for IS min and 44 degrees C for 10 min. We found that apoptosis had occurred in 7.5 and 87.0% of the cells at I week after heat treatment at 43 degrees C for 10 min and 30 min respectively. Western-blot analysis demonstrated that phosphorylation of p38 MAPK (mitogen-activated protein kinase) was markedly increased by heat treatment at 43 degrees C for 10 min. These findings suggest that activation of p38 MAPK by heat shock is associated with 3D-like cell proliferation. The results of the present study should be useful for further studies aimed at elucidation of the physiological mechanisms underlying thermotherapy and hyperthermia.