Previous studies demonstrated increased fluid intake in rhesus monkeys exposed to combined prenatal and long-term postnatal (PRE+POST) dietary deficiency of omega-3 fatty acids. Here we determined the effect of dietary deficiency in omega-3 fatty acids occurring only prenatally (PRE) or only postnatally (POST). Water intake over 24 hours, water intake in 15-minute tests, and excretion of combined urine and feces over 24 hours were all about twice as great in POST as in PRE monkeys. Neither group preferred or avoided salt solutions compared to water in two-bottle tests. Serum electrolytes, total protein, and glucose were within the normal range, and both groups concentrated urine when deprived of water. Levels of all omega-3 fatty acids in red blood cells were greatly depressed in POST monkeys, while levels of omega-6 fatty acids were elevated or unchanged. These results confirm the effects of PRE+POST deficiency on fluid intake and demonstrate that postnatal deficiency by itself is sufficient for the effects.