Carcinus maenas and Necora puber were exposed to air for 72 h and 18 h, respectively, at 18 degreesC. Nitrogen excretion, blood and muscle ammonia content and blood urate and lactate content were recorded throughout the experimental emersion and following reimmersion (recovery period). During emersion, the rate of ammonia excretion was strongly reduced in both species, while urea and amine excretion were not enhanced. Blood and muscle ammonia content increased steadily, reaching 1.3 and 10.4 mmol l-1, respectively, after an 18 h emersion in N. puber. In contrast, in C. maenas, blood ammonia levels increased slightly during the first 12 h and then remained at this level (approximately 0.12 mmol l-1) until the end of emersion. Muscle ammonia content showed a non-significant increase at 12 h, after which values returned to control values (3.3 mmol l-1) for the next 60 h. Blood urate and lactate content increased in emersed N. puber, indicating strong internal hypoxia, but urate content did not increase in C. maenas until the third day of emersion. Upon reimmersion, both species released large amounts of ammonia within a few minutes. Two different patterns of ammonia release then were observed: ammonia excretion was enhanced for a further 3 h in N. puber, whereas raised ammonia excretion rates were observed for a further 24 h in C. maenas. These patterns, the recovery of blood and muscle ammonia levels and the calculated nitrogen balance between emersed and control crabs indicated that specific processes were used to manage the nitrogen overload induced by air exposure. Whereas N. puber shows little or no ability to limit ammonia accumulation in its body, C. maenas exhibits strong regulation of its nitrogen metabolism. The probability that amino acid synthesis is involved in this regulation and whether these species use metabolic depression as a survival strategy are discussed.