The metabolism of GA10 is thought to be under photoperiodic control in the woody plant Salix pentandra. However, in a recent study using 16,17-[3H2]GA19 as a mimic of Ga10, no effect of photoperiod was found on its metabolism to 16,17-dihydro-GA20 and 16,17-dihydro-GA1. To investigate if this was due to differential action of exogenous 16,17-dihydro-GAs and GAs, the effects of the 16,17-dihydro-derivatives of the gibberellins GA19, GA1, and GA1 as compared with their parent GAs, on shoot elongation in seedlings of S. pentandra were studied. 16,17-Dihydro-GA19, and -GA20 were both almost inactive, while 16,17-dihydro-GA1 induced some shoot elongation in seedlings treated with ancymidol as well as under short days. GA19, GA20 and GA1 were all able to counteract the inhibitory effect of ancymidol under continuous light, while inhibition induced by a 12-h photoperiod was antagonised only by GA20 and GA1. Thus, the growth-stimulating activity of the tested GAs is significantly reduced by 16,17-dihydro derivatisation, but the derivatives do not inhibit stem elongation in S, pentandra, as has been found in monocotyledons.