1. Acute exercise is found to increase collagen type I formation locally in peritendinous connective tissue of the Achilles' tendon in humans, as determined from changes in interstitial concentrations of collagen propeptide (PICP) and a collagen degradation product (ICTP) by the use of microdialysis catheters. However, the local collagen type I turnover response to training is unknown. 2. Nineteen young males were studied before and after 4 and 11 weeks of physical training. Microdialysis catheters with a high molecular mass cut-off value (3000 kDa), allowing the determination of PICP and ICTP, were placed in the peritendinous space ventral to the Achilles' tendon, under ultrasound guidance, in both legs. The catheters were perfused with a Ringer-acetate solution containing (3)H-labelled human type IV collagen for in vivo recovery determination (relative recovery: 79 +/- 2 %, mean +/- S.E.M.). 3. The PICP concentration in the peritendinous tissue increased in response to training (from 5 +/- 1 to 35 +/- 5 microg l(-1) (4 weeks), P <0.05) and remained elevated throughout the training period (28 +/- 6 microg l(-1), 11 weeks). Tissue ICTP only rose transiently with training (from 2.2 +/- 0.1 to 2.8 +/- 0.2 microg l(-1) (4 weeks), P <0.05, and 2.5 +/- 0.2 microg l(-1) (11 weeks), P > 0.05 vs. basal). Plasma PICP was unchanged whereas plasma ICTP declined by 17 % in response to training. 4. The findings indicate that physical training results in an increased turnover of collagen type I in local connective tissue of the peritendinous Achilles' region. Early in the process both synthesis and degradation are elevated, whereas later, the anabolic processes are dominating causing a net synthesis of type I collagen in tendon-related tissue in humans.