To summarize, we investigated urea, one of the crucial biomolecules, under various solution conditions using ZULF NMR. We demonstrated that the compound can be readily detected in water by zero-field NMR, modifying spin topology via an chemical-exchange procedure. Our results can be extrapolated to other biomolecules with similar structures (e.g., amino acids) opening the technique for various biochemical research. We also re-ported that the J-spectra of complex molecules, such as urea isotopologues, can be clearly interpreted by identifying simple subgroups in the system and analysing their energy structures independently. All the experimental results are supported with simulations, supporting our theoretical interpretation. In turn, the work implies the ability of future in vivo/in vitro biochemical investigations of complex molecules. For example, such studies will be possible in biofluids (e.g., blood, urine, etc.) with a high concentration of water, which, in the conventional 1 H NMR, forms a challenge. Specifically, one of significant clinical analysis methods, the quantification of urea in urine and blood, will be a subject of our future research. In this context, however, it should be noted that in the presented study, we worked with highly concentrated (5-8 M), isotopically-enriched urea solutions. Even with such high concentrations, low thermal prepolarization, provided by the 1.8-T magnet, results in low amplitude of ZULF NMR signals. To overcome this limitation, zero-field NMR may be combined with hyperpolarization methods such as parahydrogen-induced polarization (PHIP), , signal amplification by reversible exchange (SABRE), dynamic nuclear polarization (DNP) , etc. As these methods are limited to a few suitable molecules, in our future experiments we plan to address the problem by the exchange-based polarization methods: SABRE-RELAY and PHIP-X that provide wide applicability for diverse biomolecules .