Plants of two clonal thymol-type thyme selections were grown under natural light and natural light supplemented by a PPF of 200 mu mol m(-2) s(-1), provided by HPS lamps at 50%, 70% and 90% soil water amount (SWA) of field capacity in a greenhouse. We investigated the variations in net CO2 assimilation rates (Pn), dry matter and volatile oil accumulation and their relationships during different development periods. Pn varied between 5.88 to 15.80 mu mol CO2 m(-2) s(-1) under natural light and from 11.93 to 28.04 mu mol CO2 m(-2) s(-1) under supplemental light. The highest Pn, 28.04 mu mol CO2 m(-2) s(-1), was obtained from ''Laval-1'', after 120 days of growth under supplemental light at 70% SWA. The lowest Pn, 5,88 mu mol CO2 m(-2) s(-1), was obtained from ''Laval-2'' after 40 days of growth under natural light at 50% SWA. Volatile oil accumulation steadily increased from 0.38% to 1.35% under natural light and 0.68% to 2.18% under supplemental light, with further development of the plants. Similarly, the dry matter shoot yield grew from 1.31 to 18.20 g/plant under natural light and 2.87 to 50.30 g/plant under supplemental light, depending on SWA and the time of harvest. We discovered, that mean shoot yield and essential oil accumulation over the growing period were fairly associated with Pn. There were significantly positive correlations between Pn and essential oil accumulation (r(2) = 0.64), shoot yield and essential oil content (r(2) = 0.58) and Pn and shoot yield (r(2) = 0.55).