The 1940–1946 period marked a decisive shift toward treating the unbound drug fraction as the principal determinant of distribution and clearance. Researchers emphasized how plasma protein binding limits tissue penetration and governs the availability of drug to interact with targets, laying the groundwork for a quantitative, mechanism-based view of pharmacokinetics. Early methodological advances enabled measurement of drug concentrations in body fluids, supporting basic distribution and elimination profiling and informing emerging dosing concepts. Across multiple agents and contexts, the emphasis on free drug fractions united disparate observations into a coherent framework for understanding how dosing, distribution, and excretion are interrelated. Historical Significance: This era established a foundational paradigm linking protein binding to distribution and excretion, providing the scaffolding for modern pharmacokinetic theory. By showing that only unbound drug can distribute into tissues and be cleared, it catalyzed the transition from qualitative descriptions to quantitative modeling, ultimately guiding therapeutic monitoring, dosing strategies, and the subsequent development of pharmacokinetic practice.