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Ionic liquids (ILs) belong to emerging pollutants, and their hormesis earned particular attentions on their ecological impacts at low concentrations with long-term exposure. As one representative IL, N-ethylpyridinium bromide ([epyr]Br) was chosen as the target chemical. Its reproductive toxicities on Caenorhabditis elegans were determined with multi- and trans-generational exposure arrangements, and effects on the metabolites and enzyme activities in lipid metabolism were explored to elucidate the mechanisms. In the multi-generational exposure over 11 consecutive generations, [epyr]Br at 0.53 mg⋅L-1 caused stimulation on the reproduction in F1, F2, F3, F7, F8, F9 and F10, and provoked inhibition in F6 and F11. At 53 mg⋅L-1, it caused greater inhibitions in more generations than at 0.53 mg⋅L-1. In the trans-generational influences, [epyr]Br at both concentrations caused significant residual inhibition in the first non-exposed offspring of F1 (i.e., T3). Meanwhile, the residual effects of [epyr]Br exposure at the low concentration in the first non-exposed offspring of F11 (i.e., T3') showed stimulation, and those at the high did not show significant residual effects. The effects of [epyr]Br on the lipid metabolites, e.g., triacylglycerol (TG) and acetyl-CoA (ACA), and those on the activities of enzymes, e.g., glycerol-3-phosphate acyltrans-ferases (GPAT) and carnitine palmitoyl transferase (CPT), also showed oscillation between stimulation and inhibition over generations. Moreover, close relationships were found among the effects of [epyr]Br on the bio-indicators and also between the biochemical effects and the reproductive toxicities with influences by the exposure concentrations. In T3 and T3', [epyr] also showed significant residual effects on the biochemical indicators. Summing up, the reproductive toxicities of [epyr]Br in multi-generational exposure scenario and the residual effects in the trans-generational scenario were closely connected to effects on the lipid metabolism with influences by exposure concentrations. [ABSTRACT FROM AUTHOR]