Elevated water temperature has been found to cause gonadal degeneration in fish, including the partial or complete loss of germinal elements, and might impair fertility and reproductive performance. Germ cell-deficient and even completely sterile pejerrey Odontesthes bonariensis have been found in two lagoons in Argentina, and exposure to warm water is one of the possible causes of these abnormalities. This experiment was conducted (a) to compare the histological characteristics of the abnormal gonads from wild pejerrey with those of animals exposed to heat in the laboratory and (b) to examine whether short-term pulses of heat similar to diurnal temperature variations in natural environments during summer can trigger gonadal cell degeneration in adult pejerrey. Wild fish with gonadal abnormalities were obtained from the San Miguel del Monte and Lacombe Lagoons (Buenos Aires Province, Argentina). Laboratory specimens were obtained by exposure of adult pejerrey to five thermal regimes (constant 24 degrees and 29 degrees C and 12-h cycles of 27 degrees -31 degrees, 28 degrees -30 degrees, and 28 degrees -31 degrees C) for up to 16 wk. Germ cell-deficient specimens for histological comparison with wild animals were also obtained by exposing larvae and juveniles for 8-12 wk to 29 degrees C and rearing until they became adults. The histological characteristics of the abnormal gonads of wild pejerrey closely resembled those of fish partially or completely sterilized by high water temperature in the laboratory. The results indicate that fluctuating (high) thermal regimes triggered germ cell disappearance in a manner comparable to a constant temperature of 29 degrees C. These results support the notion that high temperature during unusually warm summers might trigger germ cell degeneration and could be the cause of the observed gonadal abnormalities in wild pejerrey.