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The medaka extended one-generation test (MEOGRT) was developed as a multigenerational toxicity test for chemicals, particularly endocrine-disrupting chemicals. Briefly, 3 generations of Japanese medaka (Oryzias latipes) are exposed to a chemical over a 20-wk period: 3 wk in the parental generation (F0), 15 wk in the first generation (F1), and 2 wk in the second generation (F2). The present study reports the first MEOGRT results concerning branched isomer mixtures of 4-nonylphenol (NP). Adult F0 medaka exposed to NP at 5 actual concentrations (1.27, 2.95, 9.81, 27.8, 89.4 mu g/L) were unaffected in terms of reproduction, although vitellogenin in the male liver was increased dose-dependently at concentration of 2.95 mu g/L and higher. In F1, in contrast, total egg (fecundity), fertile egg, and fertility decreased as NP increased; lowest-observed-effect concentrations (LOECs) for total egg, fertile egg, and fertility were 1.27, 1.27, 27.8 mu g/L, respectively. In F1, but not in F0, secondary sex characteristics (i.e., anal fin papillae in males) were suppressed at 27.8 mu g/L NP. Vitellogenin induction in adult male fish was slightly weaker in F1 than it was in F0, however. Gonadal sex abnormality and sex reversal occurred at 27.8 and 89.4 mu g/L NP in F1 subadults. At 89.4 mu g/L NP, all genotypic F1 males in breeding pairs had female phenotype, and some even demonstrated spawning. Concentrations of NP lower than 89.4 mu g/L did not affect F2 survival or hatching. The highest detected NP level in environmental freshwater in Japan was approximately a half of the LOEC (1.27 mu g/L for F1 fecundity); in other countries, however, environmental concentrations above the LOEC are reported, suggesting that NP may be affecting fish populations. Environ Toxicol Chem 2017;36:3254-3266. (c) 2017 SETAC