There are several reports indicating that parathyroid hormone (PTH), besides inducing the formation of cyclic adenosine monophosphate (cAMP), also causes an increase in cytoplasmic free Ca2+ ([Ca2+]i) in osteoblasts, and it has been speculated that both of these second messengers are necessary to mediate PTH-induced bone resorption. In the osteoblastic cell line MC3T3-E1, bovine PTH 1–34 (10 nmol/l-1 μmol/l) stimulated cAMP formation but did not cause an increase in [Ca2+]i in adherent single cells (basal [Ca2+]i=151±5 nmol/l, mean±sem; N = 98). In contrast, subsequent addition of bradykinin (1 μmol/l) resulted in a transient increase in [Ca2+]i from a basal level of 155±11 nmol/l to a peak value of 351±60 nmol/l (N= 14). When the PTH challenge was followed by the addition of thrombin (10 U/ml), the latter induced a transient rise in [Ca2+]i from a basal level of 173±12 nmol/l to a peak at 341±33 nmol/l (N=20). Primary cultures of human osteoblasts were obtained from trabecular bone. These cells were also PTH-responsive in terms of cAMP formation. On the other hand, human PTH 1– 34 (100 nmol/l) did not affect [Ca2+]i in the isolated human osteoblasts, while bradykinin (1 μmol/l) caused a transient increase in [Ca2+]i (from a basal value of [Ca2+]i at 154±10 nmol/l to a peak value of 757±147 nmol/l within 30s;N= 16). Neither in the human osteosarcoma cell line SaOS-2 (basal value of [Ca2+]i at 94±10 nmol/l; N = 24), nor in the rat osteosarcoma cell line ROS 17/2.8 (basal value of [Ca2+]i at 85±9 nmol/l; N=9) was any effect of PTH (0.1 nmol/l-1 μmol/l) on [Ca2+]i demonstrated. In conclusion, we were unable to detect any effect of PTH on [Ca2+ ]i in single MC3T3-E1 cells, in isolated human osteoblasts, in SaOS-2 or in ROS 17/2.8 cells, whereas accumulation of cAMP was always seen. This indicates that cAMP is the major second messenger for PTH in osteoblasts.