'Waldmann's Green' leaf lettuce (Lactuca sativa L.) was grown either under 84% irradiance from incandescent (In) + 16% from fluorescence (Fl) lamps, or 100% from metal halide (MH) lamps, both at 392 micromoles s-1 m-2 of photosynthetically active radiation (400-700 nm) from 11 to 19 days after seeding. No differences in leaf dry weight, leaf area, relative growth rate (RGR) or photosynthesis (Pn) occurred after 8 days of exposure to these radiation treatments for 20 h day-1. However, a 23% reduction in root dry weight, a 123% increase in stem length and a 61% increase in stem dry weight were found with In + Fl relative to MH radiation. A photosynthetic photon flux (PPF) of 920 micromoles s-1 m-2 from the In + Fl source increased leaf dry weight by 13% and RGR by 21% relative to those at 460 micromoles s-1 m-2 from the same source. From 4 to 8 days of treatment, high PPF did not lower shoot dry gain, but did lower RGR. Photosynthesis and net assimilation rate were lower while leaf area ratio was higher at 460 than at 920 micromoles s-1 m-2 over the 8-day treatment period. When PPF from MH lamps was 400 micromoles s-1 m-2, leaf dry weight was 20% greater than if PPF was 805 micromoles s-1 m-2 after 4 days of treatment, but no differences were detected after 8 days. Relative growth rate increased by 11% during the first 4 days, but declined by 12% during the second 4 days under high- relative to low-PPF MH radiation. Lettuce productivity was stimulated by high PPF from either lamp type from Day 11 to Day 14, but not from Day 15 to Day 19 after seeding.