The effect of CO2 on ethylene-induced gummosis (secretion of polysaccharides), weight loss and respiration in tulip bulbs (Tulipa gesneriana L.) was investigated. A pretreatment with 1-MCP prevented these ethylene-induced effects, indicating that ethylene action must have been directed via the ethylene receptor. Treatment with 0.3 Pa ethylene for 2 days caused gummosis on 50% of the total number of bulbs of cultivar Apeldoorn, known to be sensitive for gummosis. Addition of CO2 (10 kPa) reduced the ethylene-induced gummosis to 18%. In a second experiment the influence of ethylene and CO2 on respiration and FW loss of bulbs of the cultivar Leen van der Mark was studied. A range of ethylene partial pressures (0.003-0.3 Pa) was applied continuously for 29 days. Ethylene caused a transient peak in O2 consumption rate during the first days after the start of application. The relation between O2 consumption rate and ethylene partial pressure could be described by Michaelis-Menten kinetics. Respiratory peaks were reduced by CO2. This inhibition by CO2 could not totally be due to competition with ethylene at the receptor binding-site, as was indicated by the use of an O2 consumption model. Pre-treatment of bulbs with 1-MCP and subsequent exposure to CO2 showed that CO2 could influence respiration irrespective of any interaction with ethylene. Ethylene and CO2 both stimulated weight loss. The effect of combined treatments of ethylene and CO2 on weight loss was at least as strong as the sum of the separate effects, which implies that competition between ethylene and CO2 at the receptor binding-site was unlikely.