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Plant growth regulators are useful for promoting plant growth and their consumption in horticultural crops is increasing day by day. However, their quality and purity are most important characteristics to estimate their efficacy. In Pakistan, the determination and quality monitoring of naphthyl acetic acid (NAA) in commercial plant growth regulator formulations by using HPLC technique is very limited. For the purpose of registration of plant growth regulators (PGRs) for marketing and for post-registration quality monitoring, a robust, simple, and accurate analytical method is needed. In this study, a high performance liquid chromatography (HPLC) method for the determination of NAA in commercial PGR samples was validated under the laboratory conditions and available resources of Provincial Reference Fertilizer Testing Laboratory (PRFTL) Raiwind, Lahore. The guidelines and values set by Collaborative International Pesticides Analytical Council (CIPAC) were considered as standard for method evaluation. The chromatographic conditions of HPLC method under study are comprised of (i) mobile phase; mixture of acetonitrile: water (30:70,v/v); (ii) C8 column; (iii) UV detection at 255nm; (iv) Flow rate of 1.0mL / minute; (v) and injection volume of 10 µL. The method demonstrated clear response linearity with correlation coefficient (r) of 0.999. In order to determine the precision, an interlaboratory comparison of NAA was conducted and percent relative standard deviation (RSD) was calculated from results received from eight (8) participating laboratories. The Horwitz equation was followed to evaluate precision of method. The experimental inter-laboratory RSD (RSDR =0.168) was within the Horwitz acceptable limit of RSDR (2.097). Method accuracy was evaluated by calculating percent recovery of analytical standard of NAA obtained from Sigma Aldrich along with its certificate of analysis. The recovery of NAA standard ranged between 99.96 to 100%. The results revealed that aforementioned HPLC method fulfilled the validation criteria of CIPAC and can be followed under laboratory conditions of PRFTL, Raiwind, Lahore for routine analysis of NAA in commercial PGRs. Additionally, the quality of commercial NAA samples marketed by different companies was assessed by following aforesaid validated HPLC method. Ten samples of NAA were collected from market and were analyzed. The results were tested for fit or unfit and compared with the company/manufacturers' claim considering the decision rule followed by the Agriculture Department, Government of Punjab, Pakistan. The NAA contents in samples ranged from 4.20 ± 0.10 to 5.13 ± 0.05%. The results showed that all the samples were fit, and it can be inferred that presently the quality of commercial NAA marketed in district Lahore is satisfactory. [ABSTRACT FROM AUTHOR]