Intermittent 1,25(OH)2D3 administration is widely used to suppress parathyroid glands in secondary (renal) hyperparathyroidism. It is unknown whether the effects of continuous and intermittent 1,25(OH)2D3 differ on vitamin D target organs other than parathyroids. Using primary cultures of rat chondrocytes (tibia) we compared the effects of continuous versus intermittent exposure to physiologic concentrations of 1 alpha,25(OH)2D3 on proliferation (radiothymidine incorporation), cell count, protein synthesis ([3H]-leucine incorporation), alkaline phosphatase activity (as a marker of differentiation) and 1 alpha,25(OH)2D3 receptor (VDR) regulation. Cells were synchronized and then exposed for variable periods to a medium containing 10% delipidated FCS and 10(-8) M to 10(-12) M 1 alpha,25(OH)2D3 (or 1 beta,25(OH)2D3 as specificity control). Intermittent (8 hr exposure every 48 hr) as well as continuous (sham washing) administration of 1 alpha,25(OH)2D3 had a biphasic effect on proliferation, that is, stimulation at low (10(-12) M) and inhibition at high (10(-8) M) concentrations. At 10(-12) M intermittent 1 alpha,25(OH)2D3 yielded higher cell counts than continuous 1,25(OH)2D3. This was seen in the log phase, which was day 3 (continuous 141 +/- 2.3% of solvent control; intermittent 185 +/- 2.0%) and in the plateau phase of growth, which was day 6 (128 +/- 2.6 vs. 169 +/- 2.7% of solvent control). Dependence on extracellular Ca is suggested by the effects of varying nominal Ca concentrations in the medium and of Ca channel blockers. Even two hours of exposure to 1 alpha,25(OH)2D3 (10(-12) M) yielded maximal activation of AP during postincubation.(ABSTRACT TRUNCATED AT 250 WORDS)