Four replicated groups of sea bass (Dicentrarchus labrax) larvae were fed diets containing an extra-high level of highly unsaturated fatty acids (HUFA) (XH; 3.7% EPA + DHA), a high level of HUFA (HH; 1.7%), a low level of HUFA (LH: 0.7%) or an extra-low level of HUFA (XLH; 0.5%) from day 6 to day 45 (experiment 1; XH1, HH1, LH1, XLH1). After a subsequent I-month period feeding a commercial diet (2.7% EPA + DRA), the capacity of the four initial groups to adapt to an n-3 HUFA-restricted diet (0-3% EPA + DHA; R-groups: XH2(R), HH2(R), LH2R, XLH2(R)) was tested for 35 d. Larval dietary treatments had no effect on larval and juvenile survival rates. The wet weight of day 45 larvae was higher in XH1 and HH1 (P<0.001), but the R-juvenile mass gains were similar in all treatments. Delta-6-desaturase (Delta 6D) mRNA level was higher in LH1 and XLH I at day 45 (P<0.001), and higher in LH2(R) and XLH2(R), with a significant increase at day 118. Concomitantly, PPAR alpha and PPAR beta mRNA levels were higher in XLH1 at day 45, and PPAR beta and gamma mRNA levels were higher in XLH2(R) at day 118, suggesting possible involvement of PPAR in stimulation of Delta 6D expression, when drastic dietary larval conditioning occurred. The low DHA content in the polar lipids (PL) of LH1 and XLH1 revealed an n-3-HUFA deficiency in these groups. Larval conditioning did not affect DHA content in the PL of R-juveniles. The present study showed (i) a persistent Delta 6D mRNA enhancement in juveniles pre-conditioned with an n-3 HUFA-deficient larval diet, over the 1- month intermediate period, and (ii) brought new findings suggesting the involvement of PPAR in the Delta 6D mRNA level stimulation. However, such nutritional conditioning had no significant effect on juvenile growth and lipid composition.