PURPOSE OF THE STUDY Mechanical stimulation plays an uncontested role in the surgical treatment of bone fractures. The effect of frequency, shape, amplitude and rise time of usually symmetrical mechanical stimuli is well known. The recovery period immediately after single pulses is potentially a critical period but has attracted little attention so far. The present study addresses the effect of extending the recovery period which may conceivably increase the efficacy of postoperative fracture management and physiotherapeutic intervention. MATERIAL AND METHODS The research method consisted of moving an isolated fragment of bone at a fixed amplitude followed by different recovery periods of between ten seconds and two hours between repeated single pulses. The amount of callus produced was observed radiologically and micro-radiologically and the material property of the healing unit was assessed by measuring the stiffness of the unit. RESULTS The result was that pulses applied after a recovery period of up to two hours produced abundant callus and a large increase of stiffness. Pulses applied after a recovery period of ten seconds produced only traces of callus and no increase of stiffness. The significant differences of mean values at 5 weeks were 74 mm2 vs. 13 mm2, and 31.5 MPa vs. 5.5 MPa, respectively. CONCLUSIONS The working hypothesis postulates a time-dependent change in tissue reaction to deformation in relation to the time allowed for recovery. Damage and consequent irritation inducing callus does not occur below the critical interval. The observed, unexpected lack of reaction to recovery intervals below 10 seconds may trigger the expectation that in postoperative physiotherapy loading at longer intervals would be preferable to activation at frequencies of locomotion. Key words: fracture healing, mechanical stimulation, callus.