Purpose: The study explores the predictive relationships of the analytical dimensions of transactions and opportunistic behavior on the transaction costs of the Brazilian pharmaceutical retail market. Originality/value: The study contributes to the advancement of empirical research on the subject and proposes indicators modeled by structural equations to simultaneously estimate the effects of uncertainty, asset specificity and frequency of transactions, and opportunism in transaction costs. Design/methodology/approach: Transaction costs were estimated with indicators related to the costs of analysis, preparation, and adaptation of contracts; these are analytical dimensions of transactions with indicators that reflect the specificity of human assets and physical/dedicated assets, technological and market uncertainty, and the recurrence of transactions between partners. The data collected via an electronic questionnaire were processed with the technique of Modeling in Structural Equations. Results: Data processing demonstrated the significant influence of the frequency of transactions on opportunism and hence on transaction costs. It also confirmed the impact of uncertainty on ex ante transaction costs and the specificity of assets on ex post costs. The results denote the relevance of analytical dimensions in the theoretical framework of Transaction Cost Theory and the importance of frequency as a catalyst for opportunism. It can be used as a parameter in strategic actions to create reliable commitments and circumvent unforeseen contractual failures in the pharmaceutical retail market.