Introduction Occupational studies suggest that exposure to insecticides may lead to hearing loss, yet effects of early-life exposure on auditory function are unknown. We examined effects of prenatal organophosphate (OP) and pyrethroid (PYR) exposure on infant auditory brainstem response (ABR). Methods 36 OP and PYR and 7 metabolites were measured in umbilical cord blood plasma from a cohort of infants in Fuyang County, China (n=234). ABR latencies for waves I, III, V were measured during unsedated sleep at 3 time points: 6wk, 9m, and18m. ABR outcomes included wave V latency, central conduction time (CCT [wave V- wave I]), and wave III-V interpeak interval (IPI [wave V-wave III]). Higher values indicate slower transmission. Composite variables were created for total OP and PYR detects. Individual insecticides were analyzed as continuous, 3-level ordinal (ND/medium/high), or dichotomous (non-detect [ND]/detect), depending on detection rates. Multivariate linear regression was used to evaluate relationships between insecticides and ABR. Results are reported as β (95%CI). Results 26 OP and PYR and 5 metabolites were detected. For OP, infants with high methamidophos (vs. ND) had longer CCT at 18m (0.11 [0.01-0.22], p=0.03) and infants with medium trichlorfon (vs. ND) had longer III-V IPI at 18m (0.06 [0.00-0.12], p=0.05). For PYR, log-3-phenoxybenzoic acid was positively associated with III-V IPI at 6wk (0.02 [0.01-0.04]), 9m (0.01 [0.00-0.03]), and 18m (0.01 [0.00-0.03]), p=0.01, 0.05, and 0.05. Infants with high cis-permethrin (vs. ND) had longer III-V IPI at 6wk (0.07 [0.01-0.14], p=0.03). Infants with medium lambda-cyhalothrin (vs. ND) had longer wave V latencies (0.08 [0.00-0.15], p=0.05), CCT (0.12 [0.02-0.22], p=0.02), and III-V IPI (0.07 [0.01-0.13], p=0.02) at 18m. Conclusions Prenatal exposure to certain OP and PYR insecticides was associated with slower auditory signal transmission in infants. Longer III-V IPI may reflect impaired auditory pathway myelination.