Cultured Acartia tonsa manifested a reproducible toxicological response through six generations. There were no statistically significant differences in the responses of F1 and F6 generations to cadmium, copper, and mercury. Cultured and field populations (parental) exposed to cadmium did not manifest statistically different response patterns. The response variability of cultured populations was less than that of field populations. The 96-h LC50 concentrations for cultured (F1) populations were 90 microgram Cd/L, 31 microgram Cu/L, and 10 microgram Hg/L. The presence and abundance of ova and unicellular glands in the circumesophogeal region were used as criteria for evaluating the nutritional condition of test populations.