Daily intake and output of water and electrolytes (Na+, K+, and Cl-) were determined for 14 days in control and Pichinde virus-infected strain-13 guinea pigs. Although water intake began to decrease 7 days after virus inoculation, total daily water output (insensible water loss, urine excretion, plus fecal water loss) had little change. However, insensible water loss alone increased markedly in the virus-infected animals. Both intake and excretions of urinary and fecal electrolytes decreased at the middle (days 7 to 10) and late (days 11 to 14) stages of viral infection. Differences between intake and output of water and electrolytes were reduced significantly during these periods. To determine a possible relationship between atrial natriuretic peptide (ANP) and urinary Na+ and water excretion over intake, we measured plasma ANP concentrations. The mean control value of plasma ANP was 24 +/- 1.0 pg/ml, and plasma ANP concentrations of infected animals increased significantly (P < 0.01) to 49.5 +/- 3.9 and 51.3 +/- 8.8 pg/ml on postinoculation days 7 and 12. Because the overall physiologic responses to Pichinde virus infection are complicated, it is difficult to postulate a single central theme concerning the pathogenesis. Nevertheless, we hypothesize that the virus-induced invisible tissue "biochemical lesions" and the consequential release of mediators and hormones were possible causes of death. Among other pathophysiologic mechanisms, the increased plasma ANP concentration may have played a role in the development of disturbed water and electrolytes metabolism during Pichinde virus infection.