There is a great urgency of detecting and monitoring myocardial fibrosis in clinical practice with the aim to improve and personalize therapy against cardiac remodelling. Hence, the aim of this study was to describe alterations in and show potential correlations between the structural characteristics and the molecular and biochemical markers of cardiac remodelling on a model of isoproterenol-induced heart failure. Two groups of 3-month-old male Wistar rats (n = 8 per group) were sacrificed after four weeks of treatment: control (placebo), ISO (5 mg/kg/day intraperitoneally). Chronic ISO treatment led to heart failure (HF) characterized by significant reduction of systolic blood pressure (SBP) accompanied by an increase in left ventricular weight (LVW) along with increased collagen content in the LV. The collagen content correlated negatively with SBP (R = -0.776, P < 0.001) and positively with LVW (R = 0.796, P < 0.001), with Col1a1 (0.83; P < 0.001) and Acta2 (0.73; P < 0.01). Moreover, the mRNA expression of fibrotic remodelling indicator, i.e. TGF-β1 tended to increase, while the level of fibrinolysis markers (MCP-1, TIMP-2, MMP) were unchanged. The plasma markers of collagen, procollagen I C-terminal propeptide (PICP) was 37.34 ± 7.10 pg/mL in control and was reduced by 42% (P < 0.05) in the ISO group and procollagen III N-terminal propeptide (PIIINP) was 1216.7 ± 191.0 pg/mL in control and was decreased by 66% (P < 0.05) in the ISO group. Surprisingly, there was no positive correlation between plasma markers of collagen, i.e. PICP and PIIINP and collagen content or molecular markers of collagen. However, both PICP and PIIINP correlated with BW (R = 0.712, resp. 0.803, P < 0.001), which was significantly reduced (by 25%, P < 0.05) in the ISO group. In conclusion, we assume that the collagen content of the left ventricle does not need unavoidably correlate with plasma markers of collagen, which might be affected by confounding factors in heart failure, such as loss of body weight, presumably associated with a catabolic condition.