Metabolic activities in two penaeid prawns Metapenaeus monoceros and M. dobsoni in two size ranges of 35–40 mm and 65–70 mm for the former, and 35–40 mm and 48–53 mm for the latter, were observed for 16 days to study the effect of unilateral (UEA) and bilateral eyestalk ablation (BEA). In both species, BEA prawns of the two size groups consumed more oxygen than the UEA prawns, leading to higher metabolic activity. Small size groups revealed more metabolic activity than the large size ones, in both species, trend for the oxygen consumption (OC) was a steadily decreasing function of time in the small size group, but a steady state condition was observed in the large size group. The small size prawns showed higher amount of ammonia excretion (AE) than the large ones. BEA prawns of both size groups excreted more ammonia than the UEA with higher ammonia quotient (AQ) values, leading to higher metabolic activity, suggesting that measures should be taken for keeping the level of ammonia in the culture to the minimum for the safety of the culture. Changes in AQ are highly age-sensitive, this being explained by higher order polynomials, unlike that observed in the OC and AE trend, with respect to time. These changes induced in the metabolic activities are discussed